4 :a-;-ril-;vit
7h- .'feii^. -1::?^:.' Uxpeî inen: for CXOS/iT J..-Î. H::;";~n.i<-_, Jni:*-vsiu< of Leicester
TV- Lew iJr.eisy i'.ioe: ircr.:.: on IJXOEAT J.A.-l. 3:\-L-^-.->, ->STIC *<;;< Working Group, Leiden
Low briefly :'po^trû£copy with EXOSAT J.,i..\\ Slceïei; Cosmic hay Working Group, Leiden lun.-.r C.- ii : a: I':n of Point X-iay Sources
Optical iJer.-ificatio:. of X-rav sources ."f./. P^>:;4- ••ij .L.si;aL Greeriuie'-i Observatory lder:tH::ôt."-. of »ray Sources with Radio Dbiecr: The Precise Location of •ray Sources ard their Identification, with Infra-red Objects •\ •X-chialini-iHl'Jorth,
Pulsating ar.d Binary \~rv: Sources E.P.J, van den Heuvei, University of Amsterdam and Brussels Source Structure via the Occultation Technique
A.C. ?abiar.t Institute of Astronomy, Carnbridae
Spâtiai Structure in Galactic X-ray Sources C.A. Ilovazshj, Meudon Observatory
The Structures of Extragalactic X-ray Sources M.S. longair, Itullard Radio Astronomy Observatory, Cambridge
X-ray Halos H. Spiegelhauer and J. Trumper* University of Tubingen
Proposals for tl.e DCOSAT Observation Programme and Choice of Orbit F..D. Andres&i, F.5R0 The experiments to be carried will be funded by LTr:", -Î:.-.: will : •• produced by a small number of research groups, while the ;;-.-': It i~. . •. will be distributed among a much wider comnurii* v> fnLb-v.'ir.^ •:.-_• selection of proposals for -he observation program--:-.
In 1974 '•he Scientific Programme Board approved thf- :i--:^:"i ;• : experiments satisfying the principal mission ob^ec" Lve.- à.:.: - .• study of some experiment options which would provl:« .=:. ir.3p:..r capability and spectroscopy in the low energy rang-' ar. I .ir. .•:•"••:. |.. of the upper limit of the range 10 iiO k-V,'.
In order to provide the European ^cientiric Conr.u:1.if." •.•;i':!i ::.-• latest information on the payload composition and ir. CM-iei : :> : • confident that all aspects of the scientific mission bad '.seer. -J:'--:. into account, it was considered that a colloquium weld prcvi-j-- *::•? most suitable forum for an exchange of views. The ecilo-.u lum entitled 'X-ray Astronomy and Related Topics' was held a' EST:!.', Noordwi]V, Holland or. February 25 and ?.<ô 1975, IT. spite of ! i~ ••-•: short notice, the colloquium was attended by some --.eventy s^i-.-r.-l .: encompassing the whole spectrum of astronomy.
Since the colloquium, the Scientific Programme Board h.;s .jppi •"-•-: •. inclusion of the imaging telescope and high energy oprior.s a:.: • ;.• choice of th* 'northern orbit', which places the emphasis for •...- occultation mode on galactic X-ray sources.
All speakers are thanked for their great efforts prior '-- an: :.:::. the colloquium and for providing the texts of their talk:; in ;•-•-:: time to Dr. R.D. Andresen» who was responsible for 'h^ ;.r-<-.i •:-.'. | • r- -.
ariar. J. Taylor jH ;>;,-:tic X-rH.y bour-es.
J. T . C'j'nanp.
Urnven'itv Coll pr<* tnndon, r.ullara Cimnp Sf'en^0 L-ibcrr; ;iolmb'jr,v bf. Mary,
Dnrk mf-| Surrey, Lr:r"lar ABSTRACT The Uhuru catalogue sources may be classified bv ^-aj actio latitude into galactic and extra galactic populations. The galactic sources include a group of iii^Jjl.v luminous objects near the galactic renter, trie compact X-ra.y sources in binary systems «n;i tne X-ray emitting sunernova remnants, T-ie?re three cntegorie? of X—ra.y soufre are discusser, and ::npo cinr-y r.nurnc categories are bripHy rpferr"'! U>. ". IHTaOPUCTION fOiiOKiru. t:.e iauncninf of tne unuru an^ CorermcuF satellites, approximately 170 - idJ X-ra,y sources are now known to (-XJHI. While more recent observations by tne instrument? on UK-1, surest tnat a minority of X-ray p' •j.rzf^ may be transient in nature, we will for tne purr-osf;; of tn i R review continue to classify sources bv r--31 ••>."* ••^ 1st itMde. The Uhuru sky map, in falactic co-ordinates, is snown in I'i^rure 1. The large populat ion (about 100 J of bright galactic sources in appar ent at latitudes below 20 . It has been suggested by MatiLsk.v et al (1973) that with a few obvious except ions, sucn as Sco X-1 and. Her X-1, sources with b"1 20° are probably extra-galactic. A log N - ioç S plot prepared by lïiatilsky et al (1973J from the Unuru catalogue is given in Fig^ire 2. The faint nigh latitude sources fall on a line of slope - 1.34 + 0<,?0, a value which is consistent with - 1.5. These extra-galactic sources are the subject of another review (Lewin ) wnile their possible relationsnip io extra-raiacitc sources is discussed by Longair ( ). The low latitude sources form two groups in the log N - le*- s Dlot. X-ray sources of larçe flux appear to constitute a group of objects with a slopp of -1.5 ,1 ;r. r';rurr- ',-•. Tn ir. va • \^ : :-. *.riO <.- u '. u'. a r : L:«' l'or^-j tic u-. .-. Trie X-r«.v ::nectra of tner-'p rcurcf-" '\'iov. -'? <:ctrjr.cn r.cr.- vnv':r>r low rvarçy cut off >-;i Î n n , if :i'jp :.:• p.bscrr»" : ' "". by ' nterst*»1 inr i^?, reouire;-' trip orvets to be at tt distancr- of ''v.1 - U Kpc wnicn wo "J i <-* r .MCÇ T r- err ; r. T:JR reriori of the ralactic centre. T'^ir 1 urr. • r.o-- \ t ,•• WC^J' I - , -, -3^ -1 tnen De al.jit 1w errs sec In t'ip follovjjnp sectior:--, we w: 1 ; brirfly niscui-r the falfictic center sources and trier, examine t-r:e COTII'^Ï object binary sources in greater detail,, Tne cln^s of X-ray emitting supernova remnants LS not représenter: significantly in FArurp ?. and will be d - <-"u:.^en s^pirateiy. Finally some brief remar.Ks about sources in /'lcbu'.ar clusters and transient X-ray sources w,!' be presented. 2. Tne Galactic Centre X-ray Sources Prior to the launchinr of IJtiaru, X-ray astronomy was carried out almost entirely witn rocket borne instru ments, Durinr this phase of tnp subject, a rmup oï about ten sources in the general director, rf tne /-alactir center attracted considerable attention. Amonf tne most notable observations of this region were carried out by oennopper et al (1970) who located several of the X-ray sources with a precision of 1" - ?'. Kownv*>r . ; - *ir;;'t .- ' •' '• à'j:i* ' T '•' O :" "" •".' ~'Jt;* n-rr^rt.'' tO l,n('RP i-h.:"c-- : •.-.( r- net FV-CV.^FÏM'J : 1. Wh 1 * " m rotr-T-porrt t.n::- • v f.yr'ii'y s-.i*--* ^ PI n^ i 1' t.n» nou^noe- rpaU.v ar^ rinse tc tnp r:-ln.ciio center, infra red or radio ;-.ind:cia+e ch.ïerf- n;-.vr also r.'.i been lorRtPdo Trie most accuratp nonitional information for ore of im*;-e rources h:^ b*-pn obtained by Davison p+ a1. tl975) wno used tne KSSL instruments on the Conernicun satellite to obtain an error box of ^" by 7" for tiiP source 9x^-1 ( 3Ul','tJ^-2ci). Tnis observation was narrid ^ut by tnearis of the lunar occultation technique. Firure H from the work of Davison 0973} snows the size arui location of tnp 9^ error ellipse to^etner witn the POPXLÎOIÏS of a number of nearby optical and radio source?. Unfortunately even witn an error box of this size, no compelling candidate object emerges. This su^T^Fts tnfit positional data of few arc Fécond precision will eventually be reouired for identifications in this region. Because of the low energy cut off which is observe"* in tne spectra of the galactic center sources, X-ray optical systems will operate at a disadvantage and observations using lunar occultations or modulation collimators may be essential for position letsrmir.at inn in tnis part of tne galaxy. ,1 A1 thoufrn , wi ~U tne ex red. W>ri of ^;JI =*"•-.•;•- rer..r.••>.:. i it ic* possible in «t trie ^ainotic X-r-'.v srurcss ma;, al ! be r-nm^ct ob.ipctr in binary Kvitpmrj, it nan oe^r pointed out bv Ranley &nd Tuohy (147^) tnut -,otn tr.p :i i;*n 1 umincsi tv source? in tne Lar,-n l'j-.:£ " >.:.ir o.'.-i-.i and the ten rala^tic center sources do not '-cijn^p. I may ne tnerefore that tries & sources will Lpr-ome ini.^r- e^tin* a?iï:n in tr;^ f'i+ur*1. F^r tnr moment, it RPP*;^ tnat further progress renui^ps ^esitinna: utii= o;' ; orecinion that, can only be rrov'.îed by tc.f 1 una1" occultation trcnmque. 3. Coir, m ct A~ray Sources in binary 3y"ten-.r One of the mo.-t exciting results utt-uie! k" "; • WDS the discovery of a cLass of v^rinbi0 rniscti^ /-•-: sources wnicn. are tncufsit to be bmarv s.vf.teœs tn:-. ;. <-o a c oniric t co.iect • Thi F compact secondary can be e \ tr.p p- black hole or a neutron star. A-ray pmi?s'on is n-^ uced by the accretion of .natter from the primary st;;r onto the secondary. Tne X-ray luminosity is TY i G , jn_ uK ( 1 ;• K ' dt wnere K and R are the mass ani rauius of the ^omp^icr. object and dM/dt is the accretion rate. JvstemF of '•. sort can produce luminosities in the ranre 1j '"' - ... ,,T< ;- sec- and the source luminosities observed in both our cv.r. ralaxy ana the Large Magellanic Cloud j»e in t.m s ranre. Koriels for the ^<-cretion process involve the capture of material from a stellar wind emitted by the primary star or tae transfer of material through the inner Lagrangian noint f-om a primary star which has overflowed jtR Ro^he lobe. If the material from tne primary has a large angular momentum, it. will form a disc around the compact ob.iect. Particles will loose aneular moment um to viscosity and gradually spiral inwards gainine energy and radiating X-rays when the temperature has increased sufficiently, in the case of a black hole secondary, radiation can only arise from the hot material in the disc. For magnetised neutron stars, an inner boundary to the disc occurs at the Alfvén radius where tJT* radial ram prpssure of the gas balances the star's z.-'-.ei.j field energy density. Prom this boundary, matter r'rllnwF tiie magnetic field line to the poles of the neutron ï-.t^r wnere t radiates at X-ray wavelengths. Tne known X-rav emitting binary systems are listed ,:. tu':-.~le I. X-ray emission is observed to be non periodic i^orn n-.c-t of trie syrterns but Uen X-1 and Her X-1 exhibit Table 1. Properties of X-ray Binaries Low Energy Peak Vrlocit> Ma* s of Slmrl. Tr-n:i DIM Luiice Dinorv Period Citt-arr of X-nry X-ioy Sourer We. Vnrinliility (diiyiO (kevï (org ,ec"1) (km sec" ) Qttasi-periodic S.590D _+ 0.0009 10°7 before 3( ;mlt«,ai'>ns down One low energy 1.5 2 3. 10 i ufU-r 03.211.7 >r. t.t-uiisitioii irnnsition Cli- X-) Xun-l'cr iodic ptllStll jo;)y tluwil .12,883 + 0.0757 in (1.1 sec XI7 00-37 XI>1]-[UT iodic pulsations down 3.-112 * 0.002 2.1 - 5.5 1.7 3 . ,0M 23 * 5 ~2 to 0.] aoc .'IL'0U00-IQ N'un-pcr iodic piiiiiitiium down 8.95 + 0.02 2.5 - 4.4 1.3 1 . 10M 20 i IS «.i.7 C..« X-3 '1.812 » pc 37 2.08712^0.00004 ' 1.5 - 4.2 10 n . io 415. 1+0. -1 0.2 - 3.0 U'LÎJ 118-00) puKiUion» Hot X-*. 1.257Pa »<-c 37 i.7ui)ir>r>-o. 000002 1.5 - 3.2 5.8 ,o 11)9.2+0.4 0.2 - 1.2 i-iuir/..:)--») prUuuons Cv- X-3 1 in i'iiNiit,ii'tis 0. 1901i(i7^U.OOI)01-I 2.0 - 4.0 $c . u,* (.li.'j-m.tiri) (tin lotiit cclipsi?) Sir. X-l io'1' Son-p.-fi.idic 0.787313+0.000001 (lulOlT-l.-.J I'ott-n to I sec 1 0.0 (excludes rtni-p *) SMC X Nun-periodic \iU0U3-T3) puiautiona down 3.8-J27+0.0010 1.5 - 3,0 Gl 3 . 103fl 1 - 4 io 100 m. c . ._ __ 10 periodic ruïsation in trip 1 to T second ranre which suggests that these pysteme contain mameti^r-d neutron stars. The nature of tne compact secondary can also be estab1ished if its mass can be accurately determined. This is much more ": ikely to nappen if the binary system is identified a+ nptic1! ar well a? X-ray wavelengths. In the case of the Vye X-1 - HDE 226363 system it has been possible to assign a lower limit of 6 K to tne comprit object mass i r and R value r>h c; r* ^~-f-'' i- r.-,rrre.-t c tn«* PTrprn^e «• * black hole in the system. However the identification of optical counterparts for X-ray sources requires arc second precision in X-ray source location. This can be provided by EXOSAT either in the occultation mode or in the offset mode if an imaging telescope is available. The presence of a black hole in a binary system may also be indicated by the observation of rapid (i.e. sub milisecond) X-ray variability. The data shown in Figure 4 from the work of Rothschild et al (1974) suggest that fluctuations on this time scale do occur in the emission from cyg X-l. The envelope duration of about 0.3 sec corresponds to the ijifall time tin2 0.1 n/M while the mili second bursts corespondence to t 2 lo ra/M , the period of the closest stable orbit. So far Cyg X-l is the only system for which there is good evidence for the presence of a black hole. Although many of these sources exhibit non periodic variability, the secondary masses are as yet poorly determined while observations with very high time resolution have not yet been undertaken. While the majority of the X-ray emitting binary systems are thought to contain neutron stars, such objects are alrr.ost certainly present in the Cen X-3 and Her X-1 systems. The X-ray emission from these sources exhibit stable pulsation on the few second tine scale, eclipsing behaviour in the 3-2 day rango and finally a longer tern variability which, for Her X-1, is quasi-periodic on a time scale of 35 days. The 1.24 second pulsation in the X-ray emission from Her X-1 is shown in Figure S. The pulsation period exhibits changes of a £e- parts in 10 from month to month but the randomness of these changes makes it difficult at present to draw conclusions from them. The pulse profile and its dépendance on photon energy are affected by the details of the accretion mechanism. While it is in principal possible to deduce the geometry and location of the emitting regions from pulse profile observations (Doxsey et al (1973), Holt et al (1974)), the profile of the Her X-1 pulse has been observed to change dramatically in a time of about an hour. Figure 6 illustrates the longer term variability observed in Her x-1. The binary orbital variation with ai.? day period is evident. The incidence of extended lows is quasi-periodic but an approximately 35 'Jay "envelope" is evidently superimposed on the 1.7 day binary variation. This envelope turns on sharply at either phase o.2 or 0.7 and then decays gradually. In addition, dips in intensity are seen usually before eclipses but both the occurrence time and the flux decrease are variable. Explanations for the 35 day cycle involve the stopping of mass transfew from the primary (Pringle (1973)) or radiation pressure induced changes in the critical Roche surface (McCray (1973)). The dips may arise due to photoelectric absorption in a newly forming accretion disc. The evolution of X-ray emitting binary systems and the nature of the accretion mechanism is treated in detail by Van den Heuvel (1975) but beofre leaving this topic, it is worth mentioning briefly the recently observed accretion wake phenomenon in Centaurus X-3 (Tuohy and Cruise (1975), Pounds et al 11975), Jackson (1975)). This phenomenon has been observed both at the beginning and at the end of extended lows of this source. Data from Tuohy and Cruise (Figure 7) show a sudden drop in the source flux at phase 0,55 which is followed by a gradual recovery. The sudden drop is suggested as being due to the alignment oi an accretion tail shock with the line of sight. A substantial increase in the photoelectric absorption column is observed in the X-ray data but the column density gradually decreases as the wake crosses the line of sight. The wake density is not usually high enough to produce this effect and so its occurence indicates the presence of a large emraount of additional gas in the system due either to an increase in the mass flow from the primary or due to a short term overflowing of the critical Roche surface. The source Cyg X-3 is rather different from the other variable X-ray sources in that it exhibits a 4.8 hour period and a quasi-sin usoidal modulation. Present models suggest a close binary system with the unusual light curve being caused by a modulation of the electron scattering optical depth as the compact object orbits the primary star (Pringle (1973) , Davison and Ostriker (1974)), Basko et al have proposed an alternative to electron scattering in which X-rays from a highly luminous coa-pact source are beamed and not directly detected but instead heat the atmosphere of the primary whose emission is visible at earth. While X-ray studies are of the greatest importance in establishing the nature of the compact objects in binary systems, Lt is evident that a more complete Understanding of these systems will only be arrived at through combined observations at visible, infra-red and raoio VMVC- ]engths. In this connection, the determination of X-ray source position is of the utmost importance and this can bo done most effectively by the use of grazing incidence X-ray telescopes or by means of the lunar occultation technique. 4. Supernova Remnants Even before the launching of Uhuru, supernova remnants were identified as a class of X-ray sources. Furthermore the study of these objects requires good spatial and spectral resolution and, in many cases, good sensitivity to photons at energies below 1 keV. A number of significant advances in remnant studies have come there fore from the X-ray telescopes on the Copernicus satellite and from a variety of specialised rocket payloads. In discussing the X-ray emission from supernova remnants, we will consider the Crab Nebula, the emission from younger remnants suc'.', as Cas A and Tycho and finally the older remnants like the Cygnus Loop and Puppis A. a. The Crab Hebula Observations over many decades of the electromagnetic spectrum strongly suggest that the radiation from the nebula is synchrotron emission from relativistic electrons in the nebular magnetic field. At X-ray wavelengths, the spectium is well established as a power law and the percentage polarisation of the X-rays has, together ..ici] tie electric vector position angle, been established as identical to those at other wavelengths tKestenbauni et al (1971)). Derailed information is now required or how the pulsar injects electrons into the nebula. Recent lunar occultations of the Crab have been observed at x-ray energies by a number of groups. The Copernicus observations reported by Davison et al (1975) showed the centroid of the nebular 2-10 keV X-ray emission desplaced by 15" from the pulsar (Figure 8). 3alloon observations carried out by Ricker et al (1975) at energies above 20 keV (Figure 9) also show the displacement and, in addition demonstrate that the X-ray emitting region is about 30" by 60" with its ling direction paralell to the nebular wisps. Further progress in understanding the details of electron transport in the nebula will require high resolution X-ray mapping. b) Young Supernova Remnants Objects in this category include the Cas A and Tycho remnants and until comparatively recently, it was felt that the synchrotron emission from relatavistic electrons was responsible for the X-ray emission. However the preliminary X-ray map obtained with the Copernicus instrument (Fabian, et al (1973)) showed no evidence for the presence of a compact object at the centre of the remnant and strongly suggeste 1 a shell structure. Copernicus studies of the X-ray spectrum of Cas A (Charles et al (1975)) indicate the presence of two thermal components at temperatures of 8.5 10 K and 30.0 10 K respectively. The hot oaterial has probably been heated by the blast wave froo the explosion and by a reverse shock wave (HcKee (1974)) propagating inwards. The remnant of Tycho's supernova has not been studied in such detail as CSL A but here again there is evidence that at least part or the X-ray emission arises in a shock heated plasma (Coleman et al (1973)). For both of these objects, detailed mapping and high resolution spectroscopy are urgently reguired. c) Old Supernova Remnants In this category are objects like the Cygnus Loop, the Vela X and the Puppis A supernova remnants. For objects such as these there is little doubt that the X-ray emission arises in shock heated plasr.a swept up from the interstellar medium by the expanding blast wave. Although definitive observations of X-ray emission lines, which require the use of high resolution crystal spectrometers, has not yet. been published, the proportional counter data are strongly suggestive of a hot plasma emission mechanism. The spatial structure of the Cygnus and Vela remnants have been examined by a number of groups using both collimated proportional counters and one dimensional grazing incidence reflector systems. Extended X-ray emission whose distribution is in reasonable agreement with the current shock boundaries is found in both cases but the appearance of the shell in X-rays is by no means uniform. Variations in surface brightness probably reflect changes in the density of the interstellar medium. The Puppis A remnant has been mapped in greater detail by Copernicus, (Zamecki et al (1973)). An X-ray map in the energy range 0.5 to 1.5 keV is shown in Figure 9. While the idea of shock heated gas swept up from the interstellar medium still appears to be valid in this case, the effect of density irregularities in the interstellar medium is anmrent. The region of peak X-ray emission probably coincides with an interstellar cloud while in the south western region, the separation between the houndaries of the X-ray mi radie emitting regions suggests that the shock is expanding into a uniform purt of the interstellar medium. References chail=5, P.A-, Culhane, J-L-, Zarnecki, J.C., and Fabian, A.C. (1975), Ap. J. 197, L61. Coleman, P.C., Bunner, A.N., Kraushaar, W.L., McCamnon, P., Williamson, F.O., Kellogg, E., and Koch, P., (1973), Ap. J. 1S5, L121. Davidson, A., and Ostriker, J.P. (1974), Ap. J. 189, 331. Davison, P.J.N-, Culhane, J.L. and Morrison, L.V., (1975) Nature, 253, 610. Davison. P.J.N., Hoffman, J.A. and Morrison, L.V., (1973), Nature, 245, 60. Davison, P.J.N., (1973), Mature, 246, 90. Doxsey, R., Bradt, H.V-, Levine, A-, Hurthy, G.T. , Rappaport, S. and Gpada, G. (1973), Ap. J. 162, L25. Fabian, A.C., Zamecki, J.C., and Culhane, J.L., (1973), Nature P.S. 242, 18. Holt, S.S., Boldt, E.A., Kothschild, R.E., Saba, J.C.R. and Serlimetsos, R. 1-, (1974), Ap. J. 190, L109. Jackson, J.C- (1975) Submitted to H.N.R.A.S. Kestcnbaum, H., Angel, J.R.P.. and Novick, R. (1971) Ap. J. 164, ..«.vin, W.H. These Proceedings. Lonyair, H. These Proceedings. Matilsky, T., Gursky, H., Kellogg/ E., Tanabaun, H-, Hurray, S., ana Giacconi, R., (1973) Ap. J. 181, 753. pringle, J., (1974) Nature, P.S. 247, 21. McCray, R. (1973) Nature P.S. 243, 94. Me Kee, CF., (1974) Ap. J. 188, 335. Pounds, X.A., Cooke, B.A., Ricketts, H.J., Turner, M.J., and Elvi=, M., (1975) Submitted to H.N.R.A.S. Pringle, J., (1974) Nature, P.S. 247, 21. Rapley, C.G. and Tuohy, I.R. (1974) Ap. J., 191_, L113. Ricker, G.R., Sheepmaker, A., Ryckman, S.G., Ballintine, J.E., Doty. J.P., Downey, P.M., and Lewin, W.H.G. (1975) Ap. J. 197, L83. Rothschild, R.e. Boldt, E.A., Holt, S.s. and Serlemitsos, P.J. (1974) Ap. J. 189, L12 Schnopper, H., Bradt, H.V., Rappaport, s., Boughan, B., Doxsey, R. , Mayer, W., and Watt, S., (1970), Ap. J. 161, L161. Ti . I.R. and Cruise, A.M. (1975) H.N.R.A.S. In press. Van den Heuvel, E. (1975) Ap. J. In Press, zarnecki, J.C., Culhane, J.L., Fabian, A.C., Rapley, C.G-, Silk, R., Parkinson, J-, and Pounds, K.A., (1973) Nature, P.S. 243, 4. / / r \- ... \ w ••; ' • i-- / r THE THIRD UHURU CATALOG OF X-RAY SOURCES Figure 1. The Uburu Map of the X-ray sky. i —1— T »' X RAY SOURCE-NUMBER DISTRIBUTION CORRECTED FOR SKY COVERAGE _ ^\*V-w SLOPE—0.4 |t |<20* NISI x\ *^- ^\ y \ • - V .5LOPÊ--1.9 SLOPE--1.3*1 \ib >Z0* \ 10' S CI./MC aure 2. A log N - loq s plot of the Uburu sources (after Hatilsky et al (1973)). 1 1 • 1 O" 20' " °'r-f> \ r -S.X - v \ % \ \ + \\ Sw 22V§1 *o GX5-1 50' _ X-RAY C±5 03 X-RAY ),.-;-, 1 .RADIO) "' 95V. ERROR BOUNDS 1 1 ' RADIO : SOURCE 1 R. A. (1950.0) 17h58ni Figure 3. The X-rau and radio error boxes for 9x5-1 (after Davison (1973)}. :^h^mm^kf^ X» >« s« —7} Va ft«(EH LSU«ICH lUCONCSJI m lib (J 3M.43I JIBUTI 3II.49I 311.3IIM 3II.S3 I SECONDS AFTER LAUNCH Fast X-ray bursts observed in Cygnus X-l (after Rothschild et al (1974)). 200 300 BINS T!.c- iior :•:-! a.2.A second X-ray pulsation (after i-'jriihy a:iJ Echreier !1974)). . *:H^^-X * .. • & : : i - V- i' / L I..--J t/.<_ 2* "at i*fl i9 ?o ?< z; 2j ?• n ^ ^* ]| M I! «n '•A % : • -» -jJ y{[ Figure 6. Her x-1 x-ray data showing the 1-7 day binary vuri and the 35 day envolope. 1 Binary Phase 0-1 0-2 0-3 0-4 0-' 0-6 0-7 0-8 0-9 0-0 0-1 0-2 111- * Mid-phas•phase ccut-ofi l 4i* Vit,^ •••» Ci ON * en i 15 ON "5 25 + 1 (_> + W < . t •, o Dips 1 t Î •«••V//** n-5 12-0 12S 130 13-5 Universal Time {Days) Figure 7. A sudden drop in the Cen X-3 flux at phase 0.55 followed by a gradual recovery (after Tuohy and cruise (1975Ï). NORTH , 10" . The results of the Crab Nebula lunar occultation observed by Davison et al (1975) showing the dis placement of the X-ray emitting region. -42'30> -43TOdk «,-~\'?l -«•3d ! L tf22" . tfjtf" RIGHT ASCENSION An X-ray contour map of the Puppis-A supernova remnant superimposed on a radio map (zarnecki et al (1973), Milne (19701). Univers ty Collere LorHo;-, ivjullarci S-^ace Scienre Laboratory no ibury St. Mary, Do :inr, Surrey, England. INTRODUCTION Ariel 5 ?-na UK-6 are the 5th and 6th satellites of the cooperative program between tne United Kingdom and United States. Ariel 5 «vas launched on 15th October from the San Marco nlatform into a near earth orbit. UK-6 has i planned lauTh in 1977 from Wallops Island in the U.S.A. The Ariel 5 satellite. The Ariel 5 satellite was built by the MSD5 space facility in Portsmouth. Six experiments are carried on the spacecraft; two from the Mullard Space Science Laboratory of University College London, two from the University of Leicester, one from Imperial College and one from the Goddard Space Plignt Centre. The mass of the satellite is 134 kg and the orDit 550 km, 502.8 km 2.9° inclination. The satellite is cylindrical with dimensions 86.5 cm x 95 cm diameter, and it spins about the axis of the cylinder with a period of 10 + 2 HPK. The spin axis can be changed in steps of up to 20 by means of a propane gas jet system having a total manoeuvering capability of 6000° with the 4.4 kg of gas. JJrift of the spin axis can be up to 0.1° per orbit, this value being obtained by use of a Dipole Cancellation tëagnetorquer. Citation damping gives a value of 0.1° witnin one nour frozn toe end oT the disturbance. Experiment data is stored in a core store during daylignt parts of tne orbit. Two core stores are used ror data compression and storage, eacn has a capacity of 4096 B bit words and serve three experiments,at a combined rate of 15000 events/sec. The associative address principle is employed,eacn event causing a 12 bit store location address to be assembled from experiment, sector, time or pulsar data, and the addressed location contents to be incremented by one. Each experiment may select up to four different combinations of address data and use two eignt bit command registers for the selection of different experiment modes of operation. The Experiments .Four of the experiments A,U.D,Fj view either along or close too the spin axis. The other two are scanning experiments with viewing direction perpendicular to the spin axis in the case of experiment B and the other, experiment G has a wide field of almost + 130 centred perpendicular to the spin axis. Details of each experiment are given below:- LXPERIMENT A Kullard Space Science Laboratory University College London. An experiment to accurately determine the positions of cosmic X-ray sources. Tnis rotation modulation collimator experiment basic ally consists of three cylindrical beryllium proportional counters behind two etched stainless steel grids, and a field limiting honeycomb collimator. The two grids define a reception pattern on the sky similar to a rectangular ruling. By rotating the experiment about an axis perpen dicular to the grid the X-ray count rate from each source in tne field of view is uniauely modulated according to its position. The total signal from all sources in the field can be analysed by a combination of cross- correlation and minimum chi-squared fitting techniques to determine the positions and intensities of all of the sources in the field. The accuracy in determining tne position of a source is limited by counting statistics, and the accuracy in the knowledge cf the satellite's pointing position, which can be determined by the experiment itself if an already well determined source is in the field. Summary of experiment's parameters window are;i "k\-*i> sq cm (reometricj Counter dimensions 0.^29 mm wail tmcitness, 7o nim diameter Fl e i "! Ut V j f;W 17 decrees fuLL widtn at max imum Spatial resolution T"; arc—minutes I'ulL wiatn at naif maximum Time résolution 1 orbit (.100 minutes) normal mode, 32 seconds time moje Energy range 2.5 - 18 keV (> 10£ effCleric,/) in four gam modes, turee enerry channel resolution Background removal Inherent in design, onl.' source signal is modulated EXPERIMENT B (Leicester University) A set of X-ray detectors look out from the side of Ariel 5 through two parallel arrays of fine slits in the side of the spacecraft. The separate ax-rays of slits collimate the view angle of the detectors at any instant to form an »X' on the sky. As the satellite spins this 'X' moves around a strip of the sky 20 degrees wide and centred in the spin plane of the spacecraft. The count.r.r rate from detectors behind each arm of tne *X' increases momentarily as an X-ray source passes through the field of view. The principal aim of the experiment is to carry out a deep sky survey for new X-ray stars, galaxies, etc. To date this survey is A/ 30 percent complete and covers large areas of sky with 3 to 5 times greater sensitivity than the only previous comparable survey (on a U.S. satellite in 1971/2). Well over 100 X-ray sources have been detected including a good number which were pre viously unknown. Data taken from 10-17 November 1974, when the instrument was surveying the Milky Way, snow a total of 78 sources are detected, including 10 discoveries. Equally surprising, 16 sources seen in the U.S. survev are absent and nave ^pnarently disappeared or become very considerably fainter over a ? - 3 year period. The 19 new sources range in Drlghtness over a factor of 'jO, the brirhtest being a •transient source*, lasting only a few weeks, the first such to be seen by Arial 5 and the sixth ever. EXPERIMENT C Nullard Spare sr^nce Lauor University Cûhe«e Lonjcr., An experiment to study spectra ani temporal variability of X-ray sources. This experiment, looking alon^ tne spacecraft soin axis, has a proportior il counter detector, collimated TO have a 3-5° PWHM field of view. The detector nas a 0.003 inch Beryllium window of 100 sq cm geometric area, and is filled with Aenon +10?b Methane. It is of muiticeil construction, achieving charge particle background rejection by operating the ceils in anti-coincidence v.itr. each other. The counter output is fed to a 123 channel pulse height analyser. The experiment is sensitive to X-rays in the energy range 1.5-30 keV ( "? 10?o efficiency over this range J. The experiment collimator is offset from the s/c spin axis by 1.75°, thus causing the flux from an offset source to be modulated, but leaving the background counting rate unmodulated'. This modulation is used to descriminate between source and background during ground analysis. When recording spectral data, the counts are integrated over a complete orbii. better time resolution (down to 1 m sec resolutionj can be achieved by trading this spectral information. Summary of main parameters ïïinaow area 100 sq cm (geometric; r'iel-i oi view 1.5° ofiset from spin axis oy 1.75 . 1-5 - 30 keV ( > 10# efficiency) analysed by 12b channel PHA Enerry Kange in two separate energy ranges: 1.5 - 15 keV, 3-30 keV. 1) Coincidence rejection of charged particles between BGND removal cells of counter. 2) By use of modulation of flux from source offset from spin axis. Time resolution 1 m sec and longer (at expense of energy resolution). EXPERIMENT D (University of Leicester) This second Leicester experiment is one of the four viewing along the spacecraft spin axis. Tt is the first instrument of its type ever flown and is designed for the difficult and more speculative tasks of observing spectral lines and / or polarisation in the X-radiation from cosmic sources. For tnis purpose it employs two ref lecting crystals, of lithium fluoride and graphite, to anal yse the X-rays of each source within 1 - 2 degrees of the spin axis. Both crystals may be rotated on command to select a different spectral line for examination and detection by the associated X-ray detectors. To 'latp both :--.:,pctrcmptpr:-; ;;n- oDera'^r.;- yerff't. ,', althourh modulât i on :r. tne underlying co:-;ir.i c-ray-indeed (b;i?KfTi;r. ! J counting rate. A deUiiJpd understanding of this mciu.^ticn is gradually beinp developed to enab';p I+.*F remove1 f r\ tr tne data. EX^ERIIV!ENT F Fhysics D^pU, Impérial Collere, London. Experiment F is designed to extend tne spectral in formation on selected X-ray sources to energies in the ran^e ?6 keV to 1.P meV. A crystal scintillator :r, •-. deep active collimator well is used for the detector. Its axis is inclined a few degrees to the satellite spin axis so tnat it cones as the satellite spins. Tne count ing rate from a point source a few decrees from tne spin axis is thus modulated with the spin period. This H'-auiation is detected by dividing the spin cycle into four sectorr and analysing the different counting rate in each. In this way, the source intensity is determined from tne amplitude of the modulation. ïne main features of tne experiment are summarised below:- "Oetectnr Crystal CsHNa), 8 crn 3.3 cm DIA x 4 cm Collimator o'sl(Na) stack of 6 crystals Overall length 35 cm Shielded by 0.? cm Al housing 0.004" Al 0.4 cm ilastic Scintillator for charged particle rejection Opening Angle 8° FWO 16 ,..' (to zero response) Energy Hanpe 26 keV to 1.2 meV nominal (50 keV to 250 keV window for pulsar mode) Pulse Hei^nt Analysis 16 channels spaced logarithmically (single window for pulsar analysis mode) operating modes (a) 256 sec integration of data (bj 512 sec integration of data in 4 spin quadrants (c) Pulsar analysis node (single channel) Weight 17| lbs Dimensions 22" x 4.5" x 5" overall Power consumption 590 mW EXPERIMENT G Goddard space Plight Centre. The aim of the experiment is the continuous monitoring of almost the whole celestial sphere. Regions excluded are those occulted by the eartn. and regions close t<.; tne .ipin axis. The experiment consists of two one dimensional posit ion sensitive proportional counters aacn having a 1 CTH'" aperture placed so tnat X-rays from a sector of tin (.•e.ssm sphere falls along tne counter. The position of -irrivui of the photon is measured so an X-ray source can be positioner; with an accuracy of a few square degrees. .summary of Experiments parameters :- lergy range 3-6 keV -2 •^nsitivity 10 aco X- î in one croît Scientific Significance of Ariel 5 Although only having been in orbit for six months, th*.- satellite has begun to contribute significantly to X-ray astronomy. The distribution of galactic sources and tneir intensities determined by the ïïbiiru satellite seess to he markedly different as measured by the survey experiment a. Positions ire being refiner! by experiment ", hign quality spectra from experiment U and long term variability from experiment G. This latter instrument has confirmed tr.e occurance of absorption events at close tc zero phase in the source (Jygnus X-1 which were discovered by the 'Jcnpr- meus satellite- The identification of the first black nole is thereby made rrore certain. Tiie most significant result from UK-5 to date is unioubtaole the observation of a transient source near to the well known source uentaurus X-3- The light curve of this source is shown in fig 1 taken from the paper by Ives, Sanford, and Bell aurnell (Nature vol. 254. April I7ta 1975)- Periodicity with a regularity of 6.755 minutes was observed oy experiment U and the insert on the figure shows tne profile of this periodicity. Periodicity on the time scale of a few minutes has not been previously observed in X-ray sources and this object may be a new type of X-ray source. Tne uK-6 Satellite The University College London University of Birmingham. The main purpose of the MSSJj-iiirmingham experiment is to study discrete sources and extended features of the low energy X-ray sky in the range 0,1 to 2.0 keV. In addition, both long and short term variability of individual sources will be studied in conjunction with the Leicester experiment. 3 4 o ïï 3 I1 I S ° 0.6755-min bins Il. m \ 'Il WV * \ W* 1.0 3.0 5.0 Juliiin Dalo 1142J00-I- Itig.* Nature of the Observations The energy ranre 0.1 to 2 keV has, so far, only been studied using detectors on sounding rockets. The need to eauip the X-ray detectors with very thin ( plastic windows in order to provide adequate transmission at low photon energies has lead to considerable technical difficulties. In addition, large area thin window detectors are susceptible to fluxes of soft electrons. These part icles can give rise to events which are indistinguishable from soft X-rays, jfor these reasons, the low energy X-ray region has not so far been adequately studied. in addition a number of the experiments which nave been carried out disagree significantly on the measured intensity of low energy X-rays from both discrete sources and from the diffuse background radiation. The present experiment offers important advantages in the elimination of sources of background, such as soft electrons, which will be referred to in the next section. The main aims of the experiment are as followss- a) The study of the spectra, structure and positions of low energy X-ray sources, eg supernova remnants, extra galactic clusters. b) The extension of the spectra of known sources from men i um en^r^ipiï ("• - ' VrreV *.n ; both enlarre our uncpr -tan^inr of tnpse source;; ar.^ to styHv tnp pi'iect o: interEiel '.fir abscrnt:on on tne "aw enerry X-ray? pm;t"ea bv sources. o) Thp study nf *np lev <-.,<>r^,* ;! • f fu~r corcnor.«-:•.+ at. eneT- be'ow i' keV. The orTin of the in ffuse X-rav errt.-.oLon in not, yet well undpr" '.ou. In nart i cular tne prop^sr • inp"1"*-.:.'.ent will be use to: i) Establish the rpïa* VP importance of r~«lLictir ar.d extr-ieaiant in sour' ~ of this radiation iij Delineate L-ompicx '. "w en err;/ emission regions sunn as the riant ? i.iio loops and the c*o~oiexes of emission in Lup; - and in Gemini-Konoceros. d) Tne senrch for new t,vp< ; of low energy X-ray sources er defunct puisars. e) The study of variable >-ray sources in conjunction witn the Leicester experiment. Instrument Design The instrument consists of a set of four grazing incidence paraboloids which provide a total e-eomptric 4(1 to tne vehicle snin axis. An aperture changing system in the foca1 plane of earn ref!petor allows thp instrument firlis of view to be set m tne ranre 0.2 to 4 . Pour thin window ( l^polypropylene) proportional counters are used to register the radiation and are mounted behind the focal plane apertures, in addition various filters, eg Boron,can bp placed in front of the detector windows and it is also possible to place a shutter over each window so that the detector particle background rate maj be measured in flight. Since the counter filling gas (1 atm of Propane; diffuse continuously througn the thin elastic detector windows, it is necessary to control the gas density in each detector by means of a system which replaces lost gas from an on board storage cylinder. Since the detector gain depends exponentially on ras density, it is necessaryto control this parameter to within 0.2$ of its nominal value through out the mission. A small quantity (about 0.5 kg) of liquid propane is sufficient for a two-year lifetime. Pulses from each detector are analysed in a 32-channel pulse neir-it analyser and 32-channel source spectra may be obtained alternatively with particle background spectra with a time resolution of 60 seconds, considerably better •~ •' ::: • tren; : ;-r:o1? which may be :.czerra.ï\p : i - rrouv : coff.mr.nr.. p'ir.-i! iy an on-board a'jto-.'...rre.,.:l.:un H.v:uem allows the instrument to obtain power- spectra wit;"! rrrr'.-..- :c. -. r. tne range ù. H m^ec tc 1 msec. The combination of gra2ing Lnci.:-.?ncc- reflectors wi'n small thin window detectorp offers a number uf important advantages over larfe area thin window proportional cuur.tpr:. in particular» the use of small detertors ieaas to a very low nartjele background rate while thf r,n\u :1 area det^rtcr winô •'.*>'.-• located at the foci of the reflector?1- can eari \y be nrotected from scft elec+ron fluxes. The ieL^ctor background rates may be measured unambiguously by nlacing shutters in front of the detectors. Finally the instrument fieln of view may be varied in flight. instrument Performance If sensitivity is defined as the flux in nhotons -? -1 cm Kec r in a .riven energy ban^, that fives rise to a fluctuation of three standard deviations (}- ) in the 42 background counting rate, then for discrete sources we ma.v write C A + I ( r A otons m_2s sAE = *» B B ^B m. )> x x' A x'* * = ^ *X C7, X V t= while for the diffuse flux ? 3(N ^RA„/B) _2 _- _1 _1 photons cm sec ster i; "A l7x V* x Tn these expressions N is the number of reflectors, C-o is the particle background rate, A„ is the detector wall area, _ is tne detector background reduction factor, -2 -1 -1 I™ is the diffuse X-ray flux in photons cm sec E , is the detector quantum efficiency, ry is the reflector efficiency, Ay is the geometric aperture of a reflector, x is the system solid angle, t is the observing time and E is the energy band vin keV) for viiich the calculation is carried out. Using these expressions, the sensitivities obtained are listed in Tafcle II for an observing time of 3.10 seconds. A diffuse background flux of 20 photons cm sec- ster" E has been assumed for both energy bands. Instrument Sensitivity Geometric Field of Solid Sensitivity for Sensitivity for Diffuse Component Aperture view (2 100cm2 4° 3.6.10-3 2.7.10"3 4.0.10"3 0.12 O.f 2° 0.9.10"3 1.4.10-1 2.0.10"3 0.43 '.4 1° 0.2.10~3 0.7.10~3 l.iMO-1 1.92 6.4 Aeff= 3° cm for •l8--z8 keV Sensitivities are in h = 15 cm2 for .5 -1.0 keV signals 44 Compel ::y &xnerimcnts in view of the sma.ll numDer of observations carried out ;^1 low enerfier, :\ number of ^-.r ou TIE are planning exper iments in tms area. The University of Wisconsin will fly a iaive arpa, thin window, proportional counter on 03G-1 in ivï'j- Tnis experiment, wnich has a. larfre fixed field c:" view (^5° rWïHK) is mainly intended to study the diffuse backrrour.ri flux. Its sensitivity is comparable with thut of the instrument pronosed here but since it is a larre area system, it will suffer from the disadvan tages referred to in the previous section. The ANS and SAS-C spacecraft will carry X-ray telescopes similar to the system, proposed here but their sensitivities are considerably less and, in addition, the ANS vehicle can only carry out its observations in local daylight ind will therefore be susceptible to scattered solar X-rays. Until the launch of HEAO-A in 1973, no instrument of comparable performance will be flown on a satellite, HEAO carries lar^e area counter instruments and is not equipped with grazing incidence optical systems. The Leicester University Experiment The primary scientific aim of the .eicester experiment 4 on UK-6 is to observe periodic, aperiodic and random intpnnity variations in celestial X-ray sources, exten-iinp from tnc relatively long term to tne very fast sub- mill i second variations predicted for particular types of coraoact stellar source. In addition the X-rav specTr=: of sources over thp energy b»nd of 1.0 - 5-° keV may be determined so tnat, in conjunction with tnp KSSL experiment, spectral information may be obtained over irore tmn twc- decades in photon energy* Scientific Aims A-ray astronomy has advanced rapidly in the last few yars, especially following the increase in observinr time provided by satellites such as Uhuru and Copernicus. Perhaps the most remarkable objects discovered so far are the 'compact X-ray sources1 characterised by rapid, some times periodic, fluctuations in intensity; some of tnese sources are also known to be eclipsing binaries and it seems likely that the great majority of these compact sources will be found to reside in close binary systems. More detailed study of these sources is of considerable interest, most topically, since it seems likely tnat tney contain iiiphly evolved stars of at least nuclear densities ^neutron stars) cr even collapsed stars in the form of *blacK noles'. Direct information on tne mass, angular momentum, magnetic field, etc of tnese stars sr.a cii tne dynamics and evolution of such, close binary systems will be afforded by X-ray ani related optical and radio observations. A number of different tyoes of investigation proposed fur the experiment are outlined below; t,^J Studies of compact X-ray sources to investigate the presence of periodic or correlated burst structure on limescales of seconds down to micro seconds may indicate the dominant Jt-radiation mechanism: while synchrotron radiation may fluctuate on very short time-scales most models of thermal brerasstrahlung emission from these sources have fluctuations slower xhan 10 ms itiiacconi, 1972J. Irregular pulsed or semi-periodic emission with periods down to a millisecond has been predicted for accretion on to neutron stars or" blackholes (Pringle and Ree?, 1972). Many sources also show variations over seconds and minutes which need to be investigated further. (bj For the kn^wr. X-ra.v pulsars, the pulse snape, spertrum as a function of pulse phase, anà detailed puLse structure all need to be Investigated. In addition pulsar slowdown or speed-UD rates a^ri other secular variations can provide important information about tne sources. (r) Other pulsars discovered by radio telesr^pps mav well have weak nnlspd X-ray emission whicn may be detectable by folding- the data modulo the known period. (d) It would also be valuable to searcn supernova remnants for pulsar emission. Radio telescopes are very insensitive to short period pulsars because of pulse dispersions, and tne apparent absence of tiulsar periods oelow 3} rns may be due to observational selection effects. [e) Tne possibility of longer term changes in sources, pg variations in tne emission from eclipsing binaries with orbital phase» should also be investigated. Instrument Design xhe X-ray detectors are four Xenon gas proportional 2 counters with multi-wire anodes of total area 400 era v.:".- mechanical roi i 1 ma tors naving a lU FiVHIT field of vieiv. i,acn pro nort ion.-: 1 counter has two spctions coveiing 1 - 20 keV £.nJ 10 - ':0 KeV respectively , and a pair of 1? cnanne1 pulse height analysers provide energy resol ution. DTtr? may be rerurdeu with out processing (.ie as a time series) with samples at 500 ** s or slower; at 500 ms the store never becomes full and continuous recording takes place. Full {SA channel) cr partial energy resolution may be selected over the whole oand or any part of it. The use of an on-board digital correlator permits a 12b lag autocorrelograrr to be computed with continuous recording down to 250|*-s smaples. Tnis range extends to I6fcks by tne accumulation of a nistogram of photon intervals whicn may be converted to a true autocorrelogram on the rroun^. Prom th^se. power spectra may be commîtes by Fourier analysis or the maximum entropy method. The pulsar mode remits counts from a periodic source to be summed coherently with the period to a precision oi" 1 i*s or 1 in 10 , ar.3 up to 256 phape elements may be used. The tmoton energy spectrum and correlogram may also be found as a function of pulse phase by suitable combinations of modef. The absolute accuracy of nulsar timing, mainly limited by orbital d-o opl er shifts, will be ab^ut ^ ms in tne worst case. The> number of combinations of operating modes: ;*IK; of independent parameter selections is very larre so ttiat observations may be tailored to chan^in^r needs. Further flexibility is provided by the defprred command store which will permit sequences and loops of observations to be set up. instrument Performance The expected performance of the experiment in some typical observations of the types outlined above IB given below: (a) Periodic or aperiodic bursts of emission ,iiay be found by time series analysis or as deviations from zero in uurreiofiramso For example, a 300 Uhuru unit vith a ?0 ic modulation on a timescale of a few milliseconds rould be detected in about 4 seconds using either time or correlator modes. Variations over seconds or minutes may be observed in a nvmher of spectral regions simultaneously; searches for pulsar periodicit ies may be carried out, for example by faat-foldin^ algor ithms. A pulsar with a period of the o^der of 1 second and a flux of 0,3 nnuna unit* could be found from the data ^f a single orbit. (b) The known X-ray sources mav be investigated using pulsar mode, e/r a 64 element spectrum may be taken for any of the ô6 phase bins in the pulse shape, and correlograms ntav PJSO be found. (c) Similarly, radio pulsars may be monitored for X-ray emission synchronised to the radio period. For typical source parameters a peak flux of 0.01 Uhuru units, or 10 of that of the Crab Nebula, should be detectable with an integration time of a day. It is worth noting that all r^dio pulsars but two have radio fluxes less than 10 of that of a typical supernova remnant, i^dj Searches for pulsars in supernova remnants may be carried out as in item (a; above, or by spectral analysis of corielograms. (ej The relatively slow attitude control system of UK-b makes observations of a single source for many days a reasonable prospect, and the study of slow changes in source parameters may easily be combined r.ith other work. competing Experiments The microsecond time resolutions required for these and similar studies will not be available on any satellite tn be launched before UK-6, Uhuru, for example, samples at 0.1 second intervals, and wiiile V&-5 may be expected to find furtner eclipsing binaries, etc, its resolution is 8 minutes (except in pulsar mode for a known pulse TDeriod). A. C. 5r:nK".a>: Space .Research 121*0 viz c VJ s j:r-. ••".: 1. Introduction Tr; Astronomical Netherlands Satellite (ANS) was launched in a sun- synchronous polar orbit in august 1974. The achieved perigee and apogee heights are 265 and 112ÛO"espectively. The ANS carries an U.V.-telescope (photometry, 5 wavelength bands) from the Univ. of Groningen and four X-ray instruments, to be discussed here. The main characteristics of the spacecraft are the following: 1.1. The satellite can point, with some constraints, towards any direction within the orbital plane, with an accuracy of one arcminute. Since the orbital plane rotates around the sun with one degree per day, the whole sky becomes visible in half a year. Apart from pointing on the source, there is also an "off-set pointing mode" whereby one looks alternately on and off the source (a step of one degree can be realized within 16 seconds of time). 1.2. A second characteristic of the spacecraft is the "slow scan mode". In this mode, the instruments scan the sky with a speed of .6°/min. This is an-useful feature to map extended sources. 1.3. The on-board computer stores the observational program, controls the attitude of the spacecraft and performs some data compression. In addition, the scientific data and housekeeping data is stored in its core memory. The observing program has to be loaded every twelve hours. 2. X-ray instruments The main characteristics (energy range and sensitive area) are given in table 1. 2.1. The X-ray spectrometer consists of a cristal with a proportional counter. There are two units, aligned in such a way, that when one is "looking at a line", the other one is simultaneously measuring the background (continuum). The cristals are fixed, the energy scanning is achieved by moving the whole spacecraft. 2.2. The ASE proportional counter instrument consists of two detectors (Be-window, argon-C02 gas). Both have a collimator in front with a triangular response. There ts a slight off-set in the viewing direction so that the combined angular response curve shows a small plateau (*= 4 arcminutes). The field of view in the direction perpendicular to the orbital plane is 3°FWHM, and 14' FWHM in the plane. 2.3. The Utrecht proportional caunter (Ti-v/indow, Me-C0? cas) nas a F^nM of 1.5 in the direction perpendicular to the o-Li to^ plane, anc a field of 30" FWHV in the plane. The response cj-ve is a triangle with a flat top witn a 13 arcminute naif angle. 2.4. For the collecting mirror, a choice out of t* *IF1-.- of view can be made (30* FWHM and 60' FWHM, both circular;. 3. Results The objects that have been studied so (a*- can be divided int. twc main groups; known X-ray sources and possible soft X-ray sojrces. 3.1. Known X-ray sources In the pointing and off-set pointing mode, a number of UHURU sources have been measured. Data, combined from 2.2 and 2.3 mainly, will yield spectral information and intensity variations on time scales from seconds {in some cases m sec.) up to at most a few davs. In the slow scan mode, the Cygnus loop has been looked at and su.e preliminary date is given in fig. 1 and 2. Fig. 1 shows the Cygnus loop in optical light. The dotted lines show the scan passes over the region. The size cf the fielo of view (30*) is drawn in. The dashed lines a, b and c refer to positions on fig. 2. The scans 52 and 51 suggest the existance of a soft source near the geometrical center. The count histograms of fig. 2 are data from the lowest energy channel. 3.2. Possible soft X-ray sources A catalogue of candidates for soft X-ray sources has been generated which contain If Geminorum stars (dwarf novae), nearby stars, flare stars, supergiants, radiopulsars, binaries etc. Data from some 50 objects has been collected and analysis has only just started. Data from two flare stars YZ CMi s'd UV Ceti -ire shown in fig. 3 and 4, repsectively. Fig. 3 shows the count rate from the softest channel (.2 - .3 lee-V). The data is summed in 16 sec. periods; the actual data has a resolution of 4 sec. A period of 32 sec. is missing around 20 06m, due to a mode switching in one of the instruments. In the .6 - 8 keV detector, not hown here, a significant signal is present as well. The total flux and peak luminosities of 6 * 1 x 10 ergs/sec and 3.9 _+ .8 x 10 ergs/sec, respectively. Optical and radio coverage was available during some of the ANS measurements, unfortunately not during the period of the X-ray burst. During this measurement the source was also outside the field of view of the on-board U.V. telescope. A similar campaign was organized for the flare star UV Ceti. Fig. 4 shows the very strong burst in the optical range. Drawn into the registration is the count histogram of the lowest energy channel. Unfortunately» the X-ray measurement started when the optical flare had already passed its maximum. With respect to fig. 2, one notices the very high background. This is due to the fact that this measurement was taken with the large field of view and also in a less favourable part of the orbit. From these examples we feel that the study of flare stars should be continued with EXOSAT which is much more suited for this work. Table l. X- Ray instruments on board A.N.S. a) An X-ray spectrometer, srt to look for Si-lines Si XIII and Si XIV 1.84 and 2.00 KeV; projected area 40 cm2 (HIT - ASE) 2 b) A proportional counter 1 - 20 KeV; effective area 50 cm {ASE) o c) A proportional counter .3-8 KeV; effective area 45 cm (UTRECHT) d) A collecting mirror .2 - .3 KeV; effective area 75 cm (UTERCHT) 3i> i JP*-» S I AT^/>&5y /j; ./ f sL'V *» .v—*—« >sr S* Jl W V» lo'¥» Right •' 30 - 1 >-JT V" ! rA HJ ^ nL ! —i 1 i i , i i i I I I I 1 1— - . 19h59m 20hO1m 20h03m 20h05m 20h07m 20h09m 20h11m »- Time ..._j._j : i _.! : • I _ L... •_..;_._...' . ' ...... i.... :|: -i •"•• i I \ l :.;r- : :t~ -•-;•-: 'i i ~v ::."."Li.i.'-i..;.. "i'li : iL^!7LiZj.:.::;"""::r'rjj i 1.1 •• _,-..' ;:! !':•:-! J 1 ! :!'.;. L : . | ! :->.. 1-j.i Mfl' • iL • : : • , •'••'; i • i-K ••'"'-:-.r - ;H-; •-.-• i-:-i-i- '; "'"J""': • W*Wli !-:: -]- •.-\--ri- : ..;' -;-|;ji m...+-: •....; J *£JS .HI/ ! •rail» ^: -i '!. V ; • -.-• ...; • • IrR : > 1 / -aço- <-^--i -.;::• i • !- , . , ;. j ; '. '• . ! • .100- 1 • . •• j; i ' i . i 0! 16 01 0! '.7 41 01 19 21 0! 2: THE SAS-C X-RAY QBSERVATOPY J. E. McClintock The following is a brief description of the SAS-C x-ray observatory which is scheduled for launch on 7 May 1975. A thorough and current descrip tion has been given by Dr. William Mayer in: "X-Ray Astronomy in the Near Future", Proceedings of ESRO Coll .quium, Frascati, May 1972. Comprehensive descriptions of engineering details and program requirements are contained in the SAS-C configuration control drawings aid records avail able at MIT and APL and in the following documents: Payload Description Document SAS-C SDO 3777, September 1974 (APL) Support Instrumentation and Requirements Document SAS-C, June 1, 1973 (GSFCÏ SAS-C System Design and Interface Document 52-0-130 (APL) General Description of the Observatory The SAS-C X-ray observatory protrayed in Figure 1 is a Small Astronomy Satellite which will be launched by a Scout rocket into a low, circular, and nearly-equatorial orbit from the San Marco Base off the coast of Kenya in May 1975. The instrument section carries 170 pounds of X-ray detectors, electronics, structure, and two star trackers with which the aei>ect can be • determined to ± 10 arc seconds. The control section, which is located in the bottom half, allows the observatory to be pointed by delayed conmand at any time and in any direction to an accuracy of ± 2° or better. Alternatively, the observatory can be rotated so as to scan or dither the detectors at a prescribed angular velocity about the spacecraft which can also be pointed and kept indefinitely on station to an accuracy of ± 2°. The on-board clock provides a time base accurate to 1 part in 1010 per orbit. Scientific data are recorded at a steady rate of 800 bps on one or another of two tape recorders which are read out on command over Quito once per orbit. Solar panels and batteries provide 30 watts of continuous power to the instrument section. Instrument Section The instrument section of the SAS-C is shown schematically in Figure 2. It has a variety of X-ray measuring devices which are describable in terms of the following subsystems. 1- Rotation Modulation Collimator Detectors tMC) for precise (M0") measurement of the positions of sources in a M00 deg2 field of view around the spacecraft spin axis (+Z) direction. 2. Slat Collimator Detectors (SO for surveying the positions, spectra and time variations of sources in a 70' wide band of directions around the spacecraft equator (+Y view direction). 3- wide-View Detector for monitoring brig"*: fluctuating and/or transient sources. 4. Soft X-ray Detectors (Cone.) with parabolic reflection concen trators, filter wheel and gas flow counters for surveying the distribution of soft X-ray emission and measuring the low energy spectra of discrete sources. Equatorial view direction (+Y). 5. Intensity Comparator Detectors with neighboring 2a FWHM view fields for precise comparisons of intensity over diffuse emission regions. 6. Extended Spectrum Analyzer Detectors for simultaneous spectrum measurements from 0.2 to 50 keV. Equatorial view direction (+YJ. The X-ray measurements are supported by three auxiliary measurements 1. Star Trackers by which the orientation of the observatory is determined as a function of time to an accuracy of ± 10 seconds. 2. Pulse Height Analyzer which is a 16-channel analyzer that can be connected on command to any measurement chain for calibration and/or refined spectrum analysis. 3. High Speed Monitor that can be connected by command, to any measurement chain or to certain combinations of measurement chains to provide high-resolution timing analysis down to 10 seconds. In addition, the instrument section has facilities for gain calibration, pulse shape discrimination, and command reconfigurations which support the operation of all the X-ray subsystems. Figure 3 shows the 5a sensitivities of the detector systems for a 10 second (^ 1 day) observation of a source and a 10 second observation of back ground. Representative X-ray spectra are also shown. One Uhuru count/second corresponds to ^ 4x10"** photons/cm2-sec-keV, and a source of thio strength is detectable at approximately a 5a level of confidence in the 1-10 keV range by the slat colliinated detectors, the 2° tubular ccllimated detectors and the rotating modulation collimators in °J 10 observation. rr FIGURE CAPTIONS The SAS-C X-ray observatory. The experiment section comprises the top half of the payload, and the spacecraft, the bottom half. A front view, engineering drawing of the experiment section. The modulation collimators view up the +Z axis. The three slat colli- mated detectors are located in the center of the drawing. The low-energy concentrator system is on the right. A sandwich con sisting of a low-energy (Ti window) detector and high-energy (Xe-filled) detector is at the bottom» Medium energy (1-10 keV) detectors are located at the left and at the top. The 5a sensitivities of the SAS-C detection systems in a 10s second observation of a source and a 105 second observation of background. Several representative X-ray spectra are shown. Cone: Soft X-ray detectors consisting of reflection concentrators and gas proportional counters. Ti: 840 ug/emz titanium window detector. MC: Rotating modulation collijnators (12°xl2'4 ; 2!3 and 4Î5 FWHM) - SC: Slat colli mator detectors ft°x50° and l°x70°). 2°: 2° tubular collimated detectors- Xe: Xenon-filled, 2" tubular colliraated, high-energy detector. One Uhuru count/second or one milli Crab corresponds to ^ 4x10" photons/cm -sec-JceV. Figure 1 io-sl 1 i mini 1 i i ,] 100 Energy (kev) AI; INTRODUCTION TO THE rxosAT •-: :::.:- :. ti.j- TayJor 70 The objectives of the EXOSAT mission are the measurement of position, structural features» spectral and temporal characteristics of cosmic X-ray sources in the energy range from less than 0.1 keV to greater than 20 MeV. The_2ccultaticn_Technigue The determination of the position and diameter of celestial objects, using the lunar disc as an occulting body, is a classical astronomical technique, for experiments conducted from the ground or from balloon and rocket platforms. However the occultation strip generated from the earth surface covers only a very small fraction of the sky. In the case of EXOSAT this fraction can be increased to some 20%, if the satellite is placed in an highly eccentric orbit with an apogee height of 200,000 km (limited by stability and communications considerations) with the line of apsides perpendicular to the moon's orbital plane. The precision to which sources can be located is limited by the knowledge of the profile of the lunar limb and the position of the satellite in orbit. For EXOSAT, the limiting positional accuracy is one arc second while source structure (relative position) can be mapped to a much greater precision. Figure 1, taken from a report by the EXOSAT Mission Definition Group, shows, as an example, positional accuracy and measurement of size 35 a function of source strength for the medium energy experiment, and indicates the performance on a number of sources. The position and structure of a source may be determined in two dimensions from one occultation with the choice of the appropriate occultation path, as shown in Figure 2. In order to obtain such a path and in fact to perform occultations in a predetermined manner, the orbital period (hence the positioning of the satellite at the correct point in the orbit for the occultation) must be control- able. This is achieved through the orbit control system which provides a total velocity increment at perigee of 200 metres/second, sufficient for 50-100 occultations or one per two orbits over the two year operational life. During the occultation procedure, the experiments need be pointed with only coarse precision {arc minutes) to the source. The_Arbitrarv Pointing Mode The observatory will perform occultation measurements for only a fraction of its lifetime. The orbit chosen will have a period such that the observatory will spend 30 hours per orbit beyond the radiation belts and consequently long uninterrupted periods for viewing sources are possible. In this 'arbitrary pointing mode' the whole of the celestial sphere, with the exception of a cone of half angle - 45°centred on the sun, will be availablft for study at any time. within -i :-,'w i^gre^;;. V/ *:.î: r.--';. : " variaKe T-UÎÏ ir.~'^t .îrr rir.ir is---: ."::.; sur an re: --r • :. "~. Hr.v-v.-r îr. • ;.- w< rr^~ wil 'ino a -;t.ir tr.i-i-^r ar> '/r.-? pi:x î:. i_ v •;•.;•-: "-- iT.r-rr^: Th* .itrii tr^.L -.-r' will ":•--• s^r.rîû iv« -jowr. r- =-f h rnagr.: • •: . • -:-. will i.-..-- i.--'_fr—ij'.^.i te in a:;UM--y "-:' al"/.:- - -i;-- £•:" :.-: . The attitii'I^ of the spacecraft- -.-."ill :.e control 1": : v : r. be included. If not, ".hen th- r^'r.tir.^ ' Ir -s-c - i.- -. :•:'..'. ':••.•:•• 1 arc second in approximately ,?.;. sec^nOs Tht> observatory •.Jill i:--?- slewed frrn or.-/ -jr.:_'ce •;:; ,-.: "h- s* The mutual alignments of the olene-rits cf tr- t-yper-"---;-.: ?: vil tc better than 10 arr ninutes and the misai ignrvn*.' ..i"" :-?."_ star sensor system car: be calibrated in fli.-ht. Communication from the observatory will be in :v--ù t'.-:•= = :.-! will net be flown. Die on-board data bar. .::.-.£ ?ys"e~ -.-.'ill : computer which will allow the optinura use if th? -kips .-3:.=. rate. An artists impression of the satellite is given if, figuro j. -I I 1 H I M J 1 1 I II III) 1—I I Mill) I J M III- ;,High instrumental background -Low instrumental background VC limiting positional accuracy Miniimm measurable size ' Statistics limited Positional Accuracy and | Upper Limit on size J typical galactic - source Tigure i •J i i 11 ml. 1 I I '' Mil t it t i i t i ... 10 ' 10° Photons/cm2.s (2-20 keV) COURSE Of THE X-RAY SOURCE USEFUL OCCULTATION ZONES 'IrtuWrj,. pieu cis" 3 The Medium Energy Experiment for EXOSAT Jeffrey A.Hoffman Physics Department University of Leicester ' . jg^irv-ents for the Ma Systen The iescription of the experiment is best preceded by a reviev :., thfl scientific requirements for the KE system, each of which is followed by the resuit.in-r constraints on detector design- i .1 Overlap with LE system: this requires a response belOH 2 keV and hence -.he use of thin windows in the proportional counters. 1 .2 High Time Resolution: necessary for lunar occultations and for studies of intrinsic source variability, this requires i) largest possible are«efficiency, ii) no artificial time variations introduced by the collimator response and iii) no background variations being mistaken for sor~ce variations. 1.3 Spectral Studies: this requires i) i_aximum areaxefficiency and ii) largest possible energy range. 1 .U Observations of î-feak Sources: this requires i) maximum area x efficiency, ii) small collimator angle to limit source confusion and iii) accurate background subtraction. 2. Summary of the MS Instrument The detectors of the EXOSAT experiment consist of two main elements: the laree-area, double layer proportional counters and the collimator. The design satisfies all the original EXOSAT mission requirements and, indeed, offers significant improvements in performance over the original concept. 2.1 The detectors The medium energy proportional detectors are designed to be sensitive to X-rays from 1-50 keV. This overlaps at the low energy end with the low energy experiment and will allow EXOSAT to make observations ever more than tui continuous decades of X-ray energy. Significant features if the medium, energy detectors are listed below:- 2.1.1 The detectors are divided into two layers, the top filled with an argon-based gas and the bottom with a xenon-based gas. This allows each layer to be designed for operation over a relatively small Cone decade) energy range, assuring gond performance and minimum desipn problems. 2-'. >2 Thin beryllium -windows, 2% un, r^r^ used tn Divide •- wrj*y response dcwn to i IreV. P.I.'1. Oas absorption paths of at least h cm-atm.provide ?n'rt X-ray con version efficiency over the «hole energy range. ?-1 -Li Irregularities in the internal structure if the counter are avoided to minimise gain non-uniformities across the counter area. 2-1.5 The internal electric fisld distribution is tailored to avoid repions of weak electric field, particularly near windows. This ensures uniform pain characteristics and the fast pulses important for efficient background rejection by rise-time discrimination. 2.1.6 Anticoincidence cells are built around the sides, ends and bottom of all sensitive detector volumes,and the argon and xenon layers are anticoincidanced with each other, ensuring efficient rejection of charged particle background. The detectors will be fabricated out of a titanium alloy whose thermal expansion coefficient closely matches that of the thin beryllium windows. This reduces the stress on these thin foils, which are one of the higher risk areas of the detector design. The proposed 2$ i±m foil lies at the state cf the art for thin beryllium technology in fairly large pieces and a test program is being under' '^n to investigate the sensitivity of these windows to various levels of stress and corrosion. The gas mixture in the upper detector will be 2 atmospheres of argon-COo with a small (— 5%) admixture of xenon to supress the argon K-edge, The mixture in the lower detector will be 2 atmospheres of xenon-No with a small admixture Ç.^%) of CO- to suppress breakdown. Both layers will have a \% admixture of He to facilitate leak detection. Figure 1 shows a schematic view of ;he possible detector design (from a preliminary design study). Figure 2 shows the total effective area as a function of energy. 2.2 The c alimator 2.2.1 The design constraints for the collimators are: (x) The channel length/width ratio defines the angular field of view. For SIOSAr the combined response of ^ach complete collimator quadrant will be %° FWHïf. (5J ) Wall thickness needs to be as small as possible to minimise charerei particle interactions in the collimator walls whilst retaining adequate strength and stiffness- (iiiï Channel vi^'h needs to be small in order to support the thin beryllium window, (iv) 'Jail material must be opaque to X-rays up to 50 '-ceV. (v) Combination of nail thickness and channel vidUi should cause a minimum loss in effective detector area. The optinun collimator design for E20SA." consists of an array of sauare tubes made from thin sheets jf stainless steel with a height of 15 cm» a channel width of 2 mm and a transmission of 96%. Structural constraints require a steel thickness ->£ at least 25 urn which is also the thickness re quited to pr-tvi-te sufficient ï-ray opacity. 3ome extra loss may be expected in effective arpa from small imperfections in the collimator array. This structure can adequately support a 2< &TH berylliu. -indow pressurised to 6 atmospheres. It is also possible to make a cylindrical tubular collimator. Several techniques under investigation offer possible savings in cost and ••-•pirtih over the square collimator, but result ir. a loss of effective area -r from 2C t-- iittt. . "'. S Flal-t^p Response Tbe cnlll-iatnr is made in four movable segments, each with a h$ ••"Tirj Fi-r-iy fiel-i "f vifjv. a siie small enough tn all™ little scurce con- :: wer; f~r it.servati ->'.< of weak sources down to 1/l0 Uhuru count/sec. v:.r;,- t.'r.i: seTnt*!-,'-s pr"'vi"ies a variable flat-top response. At the smallest '.. i'. - : arc-.ir.ii'.es, beir.»r the sa.ie as the drift limits in the pointing i r-'!-Li ;,:i --f f.he «r-actjcraft, :"">r observinr p^int sources with maximum sensi- :.:•-,-. '-X. r.r.p larref. '-his is '.ic ircmin, the maximum siae of the lunar disc •••-. •':• -. v.- r: ." c •:..<-. i-ren. ien:rei: far lunar occultations "if extended .-:."• ~- •'.::p>T::"v.-i r-rj;arj'-s and e.xiraralactÂc clusters. This -ver- -»• ••.•• :--..•: - .:.- -rv ; :•: . ;*„v :>:tvreer. the n«eds of the "->ccultati ")n" an:1 •"•" •-" - ••• , •.:.-•• -.'-'• fl'i'-'rr rrizc c-'ir; ':** nntinised for each observation, .r' .• ~ i.i - .- 'ir>-'i 't:. i •*.•;.>,îTi"î :' urc'; confus: m. >;.-.-.. -• —.. • .. :i;-.c r-rir-ciplf: -f usine movable offset colli- . /-:•-;•;•:« ris?-"T TFnirir. The flat-top is the sum •'-• - t " • -.!;•.-• • .i, i-:t-;c".-rr.. Tirure .. sh-»ws two- - - v. .• - .;•- .- • •.-• ~-.; r'.v..* "f the satell-'-e projected nnto the s/.v. Ail pi ..., «.!••_- • •:<..-..,. -., ._• - :':e. o'-'^ ar--:.: of the detector-collixiator sys^fi in tn-; triir.r:iar r-v-p "•::.•><=• n->i£. Ir. ;,.,i case, four detectir-collimator MIÎ :s -ire of:--.?:, in jrt 10,-^nal axes rotaied US to the Ï-Ô axes. The resp^nce shown is the sun 1:' -ne resp^-uies •'!' tne four detectors. As the four quadrant units are rotated outwards ir.irr. tr.fir central posit-ion, the size if the flat-tip area ir. the ror.ai sys^on response increases. 'jhe maximum size flat-ton obtainable with this system has a side equal to the F.-JHM of the indiridual trianfular-resnirio- collimator;;. Thy total effective area decreases as the square of t.V; s: ie of t.!ie fiaL-t'">p ar?:i, hence the value or usinr the minimum flat.-t.ip 3\ZP r-^uirei for a -ivr-!. observation. '•^.th a cylindrical tubular cill^mat ^r, r- cko tiii>- i,.-!c fnt;r quadrants outward would no 1 oncer rive ? pri-c: ?<>ly f ; '•!hen the medium enen:.7 experinfi'!'- in ::'-'. i.vi.-j^ faint r. r.rc-T., i t ] .-, necessary to subtract, from the on-source si mal (i.e. source plus backf round'1 an off-source sifTial (i.e. bac-cnrounn orù. , whioh is nearly as strong. Although in principle this nay be none oy r • -• i r; :. i : : - •'f sat*. Hi lu ar a: : a:. 1.. fro/n the source at remlar i;tt-ervals -• >ri:w '-:••• •:".•-.,,- peri:-?, •• L.- -.-V • has three disadvantages: (i) it uses up conLr 1 ras, (ii) on-off pointing if the n«diun rr.-vr;,? -/perir.--:.'. ray :-: incompatible ~:it'r. the re~uiren;r.:..; : • •.!•• I -w—ir:t:r-""- exneriT^r.t, where bnc'^jrrmr.d prohler..- a.-*'-- -;:.i ---1 ::;";"•.-.-'•.-.' , (iii) the frequency of on-off pointinr nar\ -i-i^^r. rv\:' -^ •.'••'•.••r than the tira scale of secular ryickfr- t:-t ' var'-iL:-::... The highly eccentric orbit. ~f KJTSAT ••.•:!: r^-.-e-,-,!;- • n i. • thxaufh "dirty" reg: ar.s of the na!7i3tosnher°, r,~.rzi';.ly i:-v.i3i:ir rapi r-i" - ground fluctuations vhich ciul : r. :t >e o""T^n. -.•,<=•' ~ r ':•;• on—-'T - ••-.':• :" a reasonable frequency (probably tens of minuter n-u-ri-^ir.). It is therefore p. ops-ss ' t,- iff set half *.h-a ->>v>r~'ors -:. •- •: cimand by — ? , so that Any c-urc? b-.-in- olssrv?-: -.'.\~\ b-> :* -•" " v : r ^. - HO of view. The mechanism for this will he the same as that which tilts all four quadrants to create a flat-top response, only it will move only two of the four quadrants off the source in the background monitoring mode. The total field of viev will still not extend beyond 3 from the spacecraft X-axis, so source confusion in the background measurements will not be a problem. This system combines equal area-tijne observations of source and background, for optimum statistics, with continuous source coverage, which would be lost with on-off pointing. It will give EXOSAT a unique opportunity to make high precision observations of a great number of weak X-ray sources which would otherwise be lost in statistical noise. Optical and radio astronopy have benefited greatly from every extension of their deep sky sensitivity, and this capability should be just as important for X-ray astronomy. 3. Background and Hoise Estimates for the ME Experiment The sensitivity of a detector of a given area depends on its noise background. In EXOSAT the background is produced by the isotropic X-ray flux, charged particles interactions, possible induced radioactivity and by the X-ray albedo of the Moon or Earth during occultation studies. 3.1 Isotropic X-ray background The response of the collimator to the isotropic background does not change as the size of the flat-top is varied. Each collimator quadrant sees the sane solid angle of sky regardless of where it is pointed. For square collimators whose FWHM are small., tha solid angle viewed is FWH-r. Folding the background spectrum of HE *^(1 - 30 keV), 36E (30 - 50 keV) photons Ccm sec sr keV) into the spectral response curve shown in Figure 2 gives count rates of 9-3 (cm sec sr)~ and 2.0(cm sec sr)~ in the argon and xenon detectors, respectively. For a FWHM of 5°, the count rates are 3.2 sec" and 0.7 sec-1- 3-2 Charged particle background Vfe consider first the relatively well-known galactic cosmic ray proton and electron fluxes, as measured, for example, on Apollo XV. These fluxes are approximately ten times more intense than below the radiation belts. The Ç-sided anticoincidence protection 3hould be at least %% efficient. If the RTD circuit removes £Q% of the remaining charged particles J ] -i: --.r- i . »f the incident onarred ' . •.'-.-i.- :.;:-; .'ill prM-.ii;'- ii .^j-- • Some of the in ci-ten t. corned r.-v"' : •:.".-, -„.;' --• -i.ics ;-ir-:i r :.V--" - vails and internal structure: of the dc*=r-- r. 7K iir-^ise nunber d^i -;r. in trie wall thickness ai. i :jn the amnunt .-:' mtV-r inside tne detect.::, whicr. should be kept to a piininurt. Gamma rays scatter!.-..- i.n the detector and producing single electrons with energy within the acceptance window of the electronics are indistinguishable fr-on X-rays depositing all their energy ir-.to electrons. These charged particle produced Parana rays are nit vet.oed by the anti-coincidence or RTD circuits and constitute an unavoidable detector background, making the "\% fi-ure used above hard to irapn, =: on. Minimum ionizing particles deposit — }• times as much ener^v per unit le:itr-i: in xenon as in ar^on. Thus the enerpy '-and--KE if 1 - i0 keV in arpon and 5-^0 keV in xenon nearly oynrlap in i.he 3-r:i-,n of the charged partiel t.- spectr-r: to which each ±z sensitive. This nsar;s rousrhly the sane char<'-?- oarticle count rate is expected in each layer. L'sinp an estimated charged -2 -i particle flux of J; cm sec depositing the nroper amount of enerpy, a -2 -2 -1 count rate of ]• x 11 cm sec is predicted, although this fini re CO'JI : easily be off by a factor of two. The area sensitive to charced Darf.:cles is th= total dett-ctir area, — 10Q1 cm , vhereas only the effective ire a, ^ ?./.) • cr, , is ser.D: t.'A to the isotropic X-ray backer Wid. Thus the detect-»" noise c~»ur.:, riV: due to charged particle interactions is 1 " sec' . Tho energy deposition spectrum of ci.nr-* i participe :;ill •"•::•:. I; be flatter than the X-ray background sp'rtr'-u-, wiiich is VJ."T. is f!v.:-T- - .L r-.ost X-ray source spectra. '-Ji,~her energy CI.-JJ^.-^IS viii t:;:;s be :,..i.vi :r i..-::. 1 ?w enerry channels. Detailed study of -he i^ leer.or oack.*r-.und .-it -,;; «nerpies will be necessary to pet the best sir:.al/;i->i?* nerfomar.c'ï. '. "; h ûi eiersy electrons Another source of c .arre i particlft r.o-ls" iz. th? n«r.«tra! . .. if the detector vind->K5 oy lo:- ener.^y olt-ctr--f , «--hich, hv.-ir.;- p-ir?^- •;.r-.w. the ber/Ilium window, deposit energy in the rerion i - i ' !w/ i,-, *,;-. ••rr'1'.. 7: o.io --ill be indistinguishable from 1 - 10 ke7 X-rays converti.'.." :.*ar I'.-- tetectT window. Results from many satellites sho" f/ir.cho-ï «f J.->-„ "vrv -.-l'jctr'Tis outside the masnetosphere. Thes° oro hirhly xoi.-.otropi" v.- i:-- rviir.i.v - •.>'ir i streaminr in the anti-s"lar '.iroctior., ro tr.it : • r--..^.-'.•. ' "ill '••« st-nopJ by th" r-olli-nat-p'-. •;:.'; --'ill o-'V-r r^r.-'.-'tf -.; Ii I ~ .ij Induced radioactivity Sepeated satellite passages through the radiation belts will cause s buildup rS radioisotopes of various elements present in the satellite. Radioactivity fror1 these induced isotopes could cause noise in the detectors. Clearly, proportional detectors are not as susceptible as scintillation crystals to the build-up of intrinsic radioactivity, since the active detecting repdon contains little matter. Heavy elements are generally more susceptible than light, and it will be necessary to study all possible cosmic ray inter actions Kith cold, titanium and irontoensure that the best choice of materials for the detectors themselves has been made. Irradiation of a detector in a measured proton flux in a particle accelerator would be a desirable ground test. Post-launch study of background noise as a function of orbit position will be needed ti delineate precisely the effects of the radiation belts. Hopefully, it will be possible to identify noise components which decay with time scales small compared to an orbit, in which case observations requiring especially low noise background can be programmed to be carried out inly after the short-term induced radioactivity noise has decayed. Continual monitoring of noise is necessary to check whether there is a long-term secular increase in intrinsic detector noise, caused by long-livei radioisotopes gradually building up in the satellite. The count rate to be expected is extremely difficult t? predict. 3o Fluorescence noise during occultations When the Moon or Earth is in the detector's field of view, albedo scattering of 3^1ar X-rays from light elements in the lunar surface or the terrestrial atmosphere will become an additional source of noise. This n'ise is mainly at energies below 5 keV and will be highly correlated with the intensity and spectral index of solar X-rays. Apollo IV measurements indicate a maximum of a few counts per second from lunar albedo, which is negligible compared to the rest of the detector background. Oxygen and nitrogen fluorescence in the Earth's atmosphere is absorbed by the beryllium window. Back-scattering of solar X-rays by the atmosohere vill be important only below 2 keV and will depend on the angle of solar illumination on the occulting limb of the Earth. All occultation observations made by EXGSAT should be accompanied by measurements of lunar and terrestrial X-ray emission, at least until it i is determine o! ^r. !e: y]-.-it cor. livi... i. .."•••; -1. ! • •-! "-——.\ ir-- 'i .-:-•; J " s.i'orce. A tl:.irD-:,-;i ••:i]C>;rsta:, :i:.' " all; E^JPC?.; f r.ois- in r.'-.e is necessary r.o USA then most eff-.-ciiv-id v. Tr.i:-; ;r..-lude.i studyin spectra and tine variations of the noise ar.d thsir correlation v\\ positiin and with solar activity. This knnwledpe i-an then be 'iss-i planning th? ->>"^rvinp Tiro=ranxe and in inten.r-".i* -• data f- rj_..in the effect, of :.-;= r. -d.•:•-. I4. Expected Performance arid Sensitivity h.1 Limiting Point Sinri^e Son^itlviLy The effective area of the nediurr. «r.err.y instf'in-jn* an a of energy is shown L:\ fij^-r? 2. '.':•;:•!• ".hi: =r.d the b^ckrr^ir:-: TO in Section Î, we can comnute the Uniting .5"?nsiT,ivi ty for p-dnt-^-) detectability. The calculation as sunt* 5 1 "' ' sec ohservinr tine (- T; orbit) and K- - detectsbility. Limiting Point-Source 3er.sitivity Point Source Strength -, Energy (xeV) (cm^ sec keV)"1 Fr-i-tion if :rar> {'/iT") 1 - n 2.ï x IÙ"'' ?-r• * 10"" 10-20 1 .H x 1 •" 2.- A : '""" 20 - LO ?-?^ x i :•"" L'..:. >: i •_: iio - 50 i .0 « 1 r!- ?.? v 1 -- li.2 Time Résolutio Thp KX1SAT *f-. po-.TiTC-t. wi" 1 r-- ah"!* '.o n.k*- r-v'.« -jo-; of variable sources over a vide r:ner.~; r^-Txra. nr-yilîr.r r-r^v- -:-,'. able information or, z^-^rcc variability. I th .-.C-;.J1-„i vi ty -will -^ detection of rapid flaring (with the flare intercity ar. T !-=-r - f -r above normal) on time scales of Z-'O asec ?~r :>r, 1- ar.'i ' _;;•"•: Crab. A l£ charge in eri. s si or. fro^ • :•.- Crab will :w dev^-a-l" and a si-^ilar ohanpp in a w*»ak hut ir.'.--r*SL'r,r r • T"Q "Li-:*: ">ar. A H^t-ict-ahl^ ^ 3 hO'irr. ^br^rva-.i";^ -f tina var. \:r.l\ :.. : V. " will con tribute siiT.l f. ^antly to unco rs tan din p X-ray s-i'::i;»:, y IS» s!»l -gdJL=Lj^ -I™ !» i> is if gr—>.\t jy * 4o Jo » t » *> hn? X-Riy£n effective area vs. I Ccrt-urt*™** tltltCTOM in_i_^2' Fig. 3 Principle of movable collimators (1-dimensional) Ftal lop Wkjih 0* Max. Sensitivity at Centre LO FXVHM of Individual estimators = 0.75' Contour M«r*at = 0.05 Fig. 4 Two-dimensional response i colli'.a tors THE LOW-ENEKCY EXPERIMENT ON EXOSAT by J.A.M. Bicekcr Couinic-P,ay-V,'orki ng Group Leidon, Netherlands "Il ï. IntrûducliuK The present concept of the Low-Energy-Experiment (LEE) for EXOSAT was proposed by a collaboration of the X-ray astronomy groups at Mullard Space Science Laboratories at University College London, the Space Research Laboratory at Utrecht and the Cosmic Ray Working Group at Leiden. In the following paragraphs the major characteristics of the instrumentation and the expected scientific return will be discussed. The summary comprises both the approved baseline configuration and the proposed option of a small imaging telescope. No details on tKe technological approaches have been included. 2. Approved baseline The experiment subsystems of the experiment baseline configuration can be subdivi'ded in: (Î) The X-ray collector The X-ray collector consists of a nested set of parabolic concentrators. The main parameters are given in Table la. Table la X-RAY COLLECTOR IBASELINE) Reflection optics : Maximum diameter : 29 cm (nested paraboloids/ Overall length : 130 cm Geometric aperture 300 cm2 Average grazing angle 2.7 degrees Reflective coating : gold Cut-off energy : 2.5 keV The use of short nested parabolotds has certain advantages over a long single parabolic reflector with the same geometrical collecting area viz.: (aj The average grazing angle in a nested set of short elements is appreciably smaller than for a single long element. This largely enhances the effective collecting area at the shorter wavelengths. The effective area for a nest comprising four elements of 30 cm in length as a function of photon energy is given in figure I. (b) The angular resolution for en off-axis point source (annular image) is proportional to the length of the reflector element, A better image quality (thin annulus) is thus obtained for a set of short reflectors. FOCAL PLANE DETECTORS (BASELINE) • Pos î tîon sensi t ive proportional counter Wi ndow 1 micron polypropylene Gas layer 1- cm-atra C,Hn or 0.1 cm-atm Xe/CH. L3n8 Max field of view 2 degrees diameter Position resolution 0.2 x 0.2 mm (35 * 35 arcseconds a-e-) Nominal energy range 0.1 - 2.5 keV Background rejection rise time discrimination energy discrimination * Inflight gassupply system necessary Low-background detector Window 0-5 micron polypropylene Gas layer 1 cm-atm C,Hp cr 0.1 cm-atm Xe/CH. Field of view 2 degrees or 10 arcmin Nominal energy range 0.O7 - 2.5 keV Background rejection side veto eel Is end veto eel Is * Inflight gassuppiy system necessary • Channel photomultiplîer Field of view : 30 arcmin. or 10 arcmin. Sensitivity range : 0.015 - I keV Av. quantum efficiency : \0% * No gassupply system necessary (î i) The focal plane detectors Three different detectors, to be selected on command, can be positioned in the focus of the reflection optics. These detectors are: (a) A position sensitive proportional counter (PSD) (b) A low-background detector (LBD) (c) A channel multiplier (CPM) The characteristics of these deteLtors are given In Table lb. The position resolution of the PSD as a function of photon energy for various gas gains is shown in figure 2. It allows for grouping of events in small resolution cells. In this way, the PSD yields a superior "signal to noise" ratio for observations of point sources or sources of relatively small angular extent ("noise" comprises the inherent detector background + diffuse cosmic X-ray background). For larger fields of view (> 30 arcmin. diameter) the LBD is slightly better The CPM is regarded as a back-up in case the other detectors would fail, it does not contain any critical components like thin entrance windows and no gassupply is necessary. The CPM can also be used in the XUV-range for broad band spectroscopy if appropriate filter materials are selected. The total effective area of the baseline as a function of photon energy is given in figure 3 for the PSD and the LBD. (îi ï) FÎIters Filter materials which will be used to improve the performance of the baseline instrument are given in table Ic. FILTERS (BASELIHE) Bro^icfhand f Î 1 cers : Eoron filter on polypropylene carrier Transmission bands 67 - 120 A < 25 8 Alumin ium fi1 ter Transmission band 1^2 - 600 A Far-UV filter Transmission band > i050 A The boron filler .^llotvs essentially for a two color study in the "Carbon window". The Boron filter blocks X-rays above 0.18 kcV in the Carbon window (see figure 4). Interstellar absorption is quite important near these energies and the two color photometry might yield valuable information on the absorption and emission of X-rays in the nearby interstellar medi urn. The UV-filter will be used to make an unambiguous assessment inflight of the detector sensitivity for UV-radiation. The transmission band of the Al-filter in conjunction with the CPH enables a search for He-M Ly-a radiation at 30*t A which most probably dominates the emission in that wavelength band. 3. The imaging telescope option Since a major portion of the EXOSAT observing time wiiI be spend in the arbitrary-pointing mode, the return from the mission can be greatly optimized by the implementation of an X-ray imaging telescope. The characteristic parameters of the proposed imaging telescope are given in Table 11. (î) The optics Within the dimensional -constraints a geometrical collecting Table 11 OPTICS (OPTION) — • Wotter 1 imaging telescope Max diameter 29 cm Focal length : 90 2 cm Geometric aperture : 110 cm2 Average grazing angle : 2 degrees Coating : go d Image quality : < 10 arcsec over 10 arcmin field IMAGE DETECTORS (OPT 1 OH) • Channel multiplier array Max field of view : 2 degrees Resolution eel 1 : 50 micron dîam (10 arcsec A.E. Average quantum efficie ncy : 7% • Position sensitive proport one 1 counter Characteristics similar to baseline'detector SPECTROSCOPY • Filters for broadband photomel ry (i.e. boron f Iter , polypropy ene filter) • Transmission grating for h gh resolution spectroscopy Period of grating : 1 micron Position : 60 cm tn front FP Spectral resolution : 25 at 0.5 keV (corrected for comatic aberration) Size of first order : 36 mm diffraction spectrum up to 300 8 area of 110 cm is achieved, by a total telescope length of 6ti cm, i.e. a 40 cm long parabolic reflector and a 28 cm loi hyperbolic reflector. Figure 5 shows the effective telescope area for three different grazing angles. An optimum effective area throughout the energy range considered is obviously obtained for grazing angles near 2 . Figure 6 demonstrates the image quality which can be achieved for off-axis radiation. A resolution of equal or smaller than 10 arcseconds over a field of 10 arcmin is attainable. H i) Image detectors The principal image detector is a channel multiplier array (CPAJ with 50 um cell size, which allows for a 10 arcsecond resolution. In addition an imaging proportional counter (similar to the baseline detector} is incorporated, since it provides a much higher detection efficiency, lower background and soue energy resolution, at the expense of spatial resolution (dbout "tC arcsec. angular equivalent). The utilization of a CPA and a PSD is highly complementsry as will be shown in the section on expected performance. (Hi) Grating spectrometer The presence of an imaging telescope allows for the use of a transmission grating for high resolution spectroscopy. The grating will be placed directly behind the telescope, the diffraction spectrum is registered with the aid of the- CPA. Aberrations caused by an equally spaced grating due to a non- parallel incident beô.Ti are mainly determined by coma. Taking this effect into account, the wavelength resolution ranges from I ~ 2.5 A over the range 10 - 300 A for a grating with 1000 1 p/mm in the proposed telescope configuration. Figure 7 shows the- effective area of the grating spectrometer as a function of photon wavelength. 4. Expected-scientific return and performance The scientific return from the low-energy-expcrimcnt in EX0SA1 can be reviewed in the scope of the two modes of operation and the principal type of measurement therein, i.e.: 1(1 (\) Source-occul t.>tïon mode (a) Positional accuracy (b) Angular extent/structure (iiJ Arbitrary-pointing mode (a) Position determination and identification of point sources. (b) Happing of the brightness distribution of extended sources. (cj Study of source spectra and absorption measures. (d) High time resolution studies of sources. (e) Spatial and spectral studies of the soft X-ray background. The performance of the LE-experiment in the occultation mode is equal or better than given in ESRO SP-87 (page 137. figure 8) to which I refer here. The performance for the various types of observations in the arbitrary pointing mode is demonstrated in table Ml. The performance figures in table III have been derived by taking into account the following types of detector background: 2 (a) shot-noise in the CPA of 1 count/cm sec. 2 (b) A cosmic-ray induced background of 0-3 counts/cm sec for the -2 2 CPA and 6x10 counts/cm sec keV for the PSD. This cosmic-ray induced background applies for a deep-space orbit (derived from Apollo XV cislunar orbit, proportional counter data). (c) The soft X-ray background folded with the appropriate instrumental response function. Figure 8A and B show the 5a-sensîtivity levels of the light bucket and the imaging telescope as a function of energy for 10 seconds observing time. Spectra of Sco X-1, Puppis-A and the Coma Cluster have been folded with the instrument response for direct comparison, they are however considered as point sources. The performance figures for mapping brightness distributions as riven in table III for Puppis and Coma-assume a source extent of *t0 arcminutes and a uniform brightness distribution. This obviously constitutes a worst case. TABLE I I I PE*F0r(M4N:E IN TriE ARBITRARY POINTIHG «ODE BASEL 1 HE IMAGING UPTION Accuracy Sens i t i v i ty Accuracy =nsi tïvîty in 10'* sec in 10** sec Position PSD 1-2 a rem in 5xl0~7 Sco (BLf* U0 arcsec 10 b Sco {PL? CPA 10 arcsec 10"5 Sco (PL) . .ghtness PSD 2 ercmîn 3xlO~3 Puppis *»0 arcsec 3x10 Puppis In distribut ion (BL) 40x^0 arcsec (PL) Ix Coma in 40x1*0 arcsec (PL) CPA 10 arcsec lx Puppis in 25x25 ôrcsec (PL) Coma no mapping on arcmin scale Broadband X/ùX=l-6 - X/A>=l-6 - photometer Grating X/AX= 25 Coronal 1ines of | spectrometer at 25 R suntype stars at 1 parsec in 3*10^ sec Emission 1ines 10'1* Puppis in IKW^ sec Temporal 10 mîcrosec 10 microsec resolution Soft X-ray PSD Large scale mapping In 10 sec mapping in 1 arcmin background not feasible cells over a field of l degree (PL) j for a uniform brightness distribution • CPA Ho mapping possible * PL = photon limited observation. ** BL = background limited observation. Table ((( clearly shows that an imaging telescope with two image detectors, i.e. a CPA and a PSD, would be optimum since: (a) The CPA is most suited for accurate positioning of starlike objects and/or bright spots in extended sources. In addition, the CPA gives the best X/h\ for spectral studies with the grating. (b) The PSD is far superior for mapping of extended sources with a relatively smooth brightness distribution, since it has a larger quantum efficiency for X-rays (one order of magnitude) and a much lower background than the CPA. (c) Studies of the brightness distribution of the soft X-ray background 'on Che arcmin scale can be performed with the PSD, this is not possible with the CPA. 5. iTnplementatzcn of the imaging telescope option In order to implement the option of an imaging telescope at a minimum escalation of weight, power and costs, the following approach has been pursued. (a) Modification of the approved baseline configuration (see section 2). Recent results on the reliability of 1 urn polypropylene windows and -further reduction of the intrinsic detector background of the PSD give good confidence that this detector alone satisfies all mission .requirements for the baseline. The use of a complex turret system with three detectors (PSD, LBD and CPMJ, selectable on command, is therefore abandoned. This yields a considerable weight and cost saving which can be allocated to the imaging mirror system. (b) Retaining of the imaging telescope configuration (see section 3). It has been demonstrated in section 4 on experiment performance that it is necessary to have both CPA and PSD present in the focal plane of the imaging telescope due to their complementary performance over the whole spectrum of observations and thus to fully exploit the capability of the imaging telescope. The inclusion of the imaging telescope does not demand more severe requirements on spacecraft structure or attitude control than those specified for the baseline instrument. One might consider to implement the CPA/P5D assembly for the imaging telescope also in the baseline instrument, which introduces redundancy for the single PSD. This would not introduce additional development costs. In summary, the preferred lew-energy-experiment configuration constitutes a modified baseline, featuring a PSD and a CPA in the focal plane and an Imaging telescope conform the description in section J. The weight penalties (in kilograms) are listed below for cases v/ith and without onboard computer (OBC). With OBC Without OBC Approved baselïne 19 17 New configuration 32 29 Reference: X-ray Astronomy in the near future, Proc. Frascati Colloquium 1972, ESRO SP-87- Figuri- copiions Figure 1. Effective aroa of a nested reflector comprising four parabolic elements of 30 cm în length as a function of incident photon energy. Figure 2. Position resolution of the position sensitive proportional counter as a function of photon energy for various gas gains. Figure 3- Effective area of the baseline instrumentation as a function of photon energy for the PSD (1 v) and UD (D.5 M). Figure h. Effective area of the baseline instrumentation with and without the Boron filter inserted in the beam. Figure 5- Effective area of the invaging telescope as a function of incident photon energy for three different average grazing angles. Figure 6. Spatial resolution of the imaging telescope as a function of off-axis angle. Cases fer a flat focal plane and an optimally curved focal plane are shown. Figure 7- Effective ?rea of the grating spectrometer as a function of photon wavelength. Figure 8A. Minimum source detectabi I i ty (5cr~level of confidence) for the light bucket assuming an integration time of ID seconds. For comparison the spectra of Sco X-1 , Puppis-A and the Coma cluster, folded with the instrument response, are given- figure 8b. As figure 8A for the imaging telescope. FIGURE 1 10-1 r __ EUCHGY ( KcV ) POSITION RESOLUTION FOR COt-LtMATED RADIATION PIO GAS MIXTURE \ \ \ 104 GAS MULTIPLICATION FACTOR I.IO^V». 3.0 4.0 EHËRSV ( KEV) 10J IIiIIH•; Geometric area 300 cm2 Counter gas icm atm propane '0.5\l polypropylene . 1 ]1 polypropelcne 102 ;101 noLL FIGUHE 3 ENERGY (keV) gracing angle (X reflective coating: gold. a. 2.0 —• a=i.5 0.5 1.0 1.5 2.0 FIGURE 5 energy (keV) I 1 i I i i 100 flat focal plane/?.-''• *ÏT S S s *r x Q) /PSi^ ^ Jr>'X ^ />• -ff/ curved focal plane diu s (ar c // /A? e 1/ / / V grazing angle -§10 If / tyj' •o • 1°5 i- ' => il 11 l 3 III / x ?-°0 o 2?5 ip J if / II il i 1 1 I I 7/ 1 10 15 20 25 30 off-axis angle (arc min.) ~i i i i i r i i i i n i i i effective area of grating spectrometer 10-' _L 10 20 30 40 50 60 70 80 90 100 110 150 A(A) FIGURE 7 -1 1—I—I I I 111 1 1 1—I I I I l 10' 1 1 1 1 1 II i 1 I 1 1 1 1 1TT light buckol imaging lelctcope PSD PSD I03 - \scoX-1 - SS^-' ~-s. - - \ \ - PuppitA /**~J"~*""N.. Coma , . \ \ >. \ Puppis A - - - - - 5 y minimum source 5(7 minimum source delectibilily deteclibilily 1 (t=10ssec) (l = 10ssec) J I I I I I I I I _1 _1 1 III i i i i t i i i 1 1 l l 1 1I 1 0.1 energy (keV) 1 0I energy (keV) FIGURE $• 10*1 LOW ENERGY SPECTROSCOPY WITH EXOSAT by J.A.M. Bleeker Cosmic-Ray-Working Group Leiden, Netherlands I H) The low-energy experiment on EXOSAT includes broad-band spectroi-'eter capability, whereas the optional imaging telescope ÏS also equipped with a grating spect rometer which allows for intermediate resolution (>./A>. = 15 -* 100) spectroscopy in the energy range below 1 keV. The scientific return to be expected from this spectroscopic feature of the low-energy instrumentation is briefly discussed in the following paragraphs. (i) Point sources Many of the observed galactic X-ray sources are thought to emit X-rays by thermal bremsstrahlung and should therefore exhibit emission lines characteristic for the radiating plasma. The fractional amount of power in the emission lines is a measure for the element abundance, the temperature and the optical thickness of the source under consideration. The continuum spectrum of Sco-X-1, for example, is fit reasonably well by a thermal bremsstrahlung function, however several ittempts to detect the suspected presence of the Fe XXV line at 6 keV did fail- This indicates that the emitting plasma is not optically thin. One of the outstanding results of the UHURU mission is the detection of X-ray sources in close binaries. Evidence for nine X-ray binaries with orbital periods ranging from 0.2 days to about 10 days has been found. The X-rays are presumably produced by accretion of matter from a large star onto its highly compact companion {i.e. a neutron star or black hole). The accreting matter might either be from a directed flow forming an accretion disk around the compact star or from a stellar wind which envelopes the large star and its compact companion. X-ray line emission is certainly expected from such material, however the lines may be considerably broadened by electron scattering in the material envelope and, in addition, by rotational and gravitational effects, ft might therefore be, that the emission lines emerge only as broad features, which cou'd explain the negative result on the existence of a pronounced Fe XXV line in Sco X-l. Spectral measurements with high sensitivity and intermediate resolution (grating spectrometer) ^re therefore nighly appropriate for observation of moderately thick plasmas. ill In addition to the emission lines, odqes due to X-ray absorption in che ci reurns le I lar envelope will occur in the source- spectra. X-rays from the compact object will photo-ionize the stellar wind or the accretion disk in its immediate vicinity. Buff and HcCray (1971*) have shown that absorption edges due to the presence of highly ionzed ion species of carbon ami oxygen if the stellar wind should be observable. The K-absorption edges of the ion species in the ci rcumstel lar gas differ significantly fror? each other, e.g. 23-3 8 (0+0), 18.5 8 (0+5), 16.8 8 (O*6), 14.3 2 (0+7j. The grating spectrometer proposed for EXOSAT has a resolution of about 1 fi at these wavelengths and can thus resolve individual ion species in the stellar envelope and in addition a neutral component in the interstellar medium. For example, the presence of a K-edrje, due to absorption by neutral interstellar Oxygen, in the spectrum of Sco X-l can be detected in 700 seconds with a 5o~level of confidenco by the grating spectrometer (assuming Brown and Gould abundances (1970))- The soft X-ray cut-otr energy is a function o*" orbital inclination and phase of the binary. Tracing the orbrta! variation of the absorption measure (edges!) might yield a soft X-ray periodicity în non eclipsing binary systems. X-ray emission from stellar coronae should ressemble the spectrum of the solar corona which shows copious line emission in the region 10-70 A. No X-ray emission from stellar coronae -12 has yet been discovered, but predicted values of 10 ergs -2 -1 -1 cm sec keV at 1 keV have almost been reached by Copernicus. Several may be found by HEAO and EXOSAT and most of their energy wl! 1 be in 1ïnes . (iIÏ Extended sources To this class belong several supernova recants (SNR) and extragalactic clusters which have jeen shown to emit soft X-rays. Spectral data on the brightest SNR's (Cygnus Loop, Veta X) indicate that the X-rays are thermal in oi igin. The emission could arise from a shock-heated envelope of sw:pt-up interstellar matter produced by the expanding remnant, temperatures in the range 2x10 - 2x10 K have been observed and a substantial fraction of the X-ray output should be in lines {especially at the lower temperatures}. Moreover the emission regions (e.g. fragmentary shell) will have low optical thickness so that the lines will not be broadened by electron -2 scattering. Doppler broadening can be of a level £A/> = 10 for the integrated source output. An example of X-radiation from an optically thin plasma with temperatures of 1.6x10 K and !?x)ij K is snown in figures )A and IB (Kato, )3/5). Observational evidence for the presence of emission lines has been obtained for the Cygnus Loop (0 VII and 0 VI11 lines, Stevens et al. 1973) and for Cas A and Tycho SNR's {enhanced Si, Hill et al. 197*0. X-ray spectroscopy on supernova remnants seems thus very rewarding and provides a tool for determining abundances of ejected matter in the case of young remnants (clues to nucleo-synthesis) and of the interstellar matter for old remnants. The extended X-ray emission associated with clusters of galaxies is not well understood, but could be, at least in part, thermal in origin (Gorenstein et al. 1973)- Again, X-ray spectroscopy provides a powerful diagnostic means to establish the nature of the emission and serves to discriminate between competing models, like inverse Compton scattering, for cluster emission and extended emission from individual radîogalaxies. The performance of the grating spectrometer on EXOSAT for extended sources will entirely depend on the brightness distribution of the source under observation. Optimum performance is only obtained when the emitting regions are point like (e.g. bright knots or thin filaments). Regions of diffuse emission give rise to a mixture of spatial and spectral features in the focal plane of the telescope and are very difficult to disentangle. For extended regions of diffuse emission, probably with a comparatively low surface brightness, the broi^rr hand spectrometer will be very useful to provide • -.TL- .-,5 in \te\] defined energy r»ands. These bands can be selected in such a way that 1 i r.e enîssîon of ion spc-cies f^orn a particular elen-ent donrnates the emission in a certain pass- band. This is illustrated in figure 2 (from Vaiana et al., 1973) A thin (0.85 un) ,ihr i n i zed paralyne filter transmits the spectrum of figure IA f 1 .6x10 K) in such a way that a significant fraction of the remaining energy is in 0 VII at 21.6-22.1 S. In case of a 5x10 K plasma, a substantial fraction of the transrtii tted energy is in Fe XVI I at 12. 1-16.8 8. ) JtitL-rnti'! ;'«)' Kù-slrv* Attenuation of X-ray source spectra by interstellar material is dominated by the He-abundance for energies below 0-53 keV and by the 0- and Ne-abundances above this energy. With the aid of the grating spectrometer the K-absorption edges of 0 and Ne can be directly measured and hence their column densities (abundances) in various directions. Observations of the diffuse soft X-ray background (< 1 keV) have revealed the p esence of emission from the galactic disk. Spectral analysis of this radiation, employing spectral data obtained through different soft X-ray filters, unambiguously shows the presence of at least two tempe-ature components, the lower being of the order of half a million degrees {Bleeker and de Korte, 1971*)- This submi 11 ion degree emission can be interpreted as a "hot" phase of the interstellar gas which also causes the 0 VI absorption line measured in the UV-spectra of several stars by Copernicus (Jenkins and Heloy, 197M- Happing of the spatial structure of this "diffuse" disk emission in the various energy bands provided by the filter spectrometer is of exceeding importance for a better knowledge of the physical state of the interstellar medium. References Bleeker, J.A.M., and de Korte, P.A.J., 197*t, Prcc. Calgary Conf. X-rays in Space. Brown, R.L., and Gould, R.J., 1970, Phys. Rev. D. J_, 2252. Buff, J., and McCray, R., 197*1, Ap.J .Letters, J_8lB, L37- Horenstein, P., Hjorkholm, P., Harris, E., and Harnden, F.R., 1973. preprint subm. to Ap.J.Letters. Hill, R.W., Burginyon, G.A., and Seward, F.D., 1971», preprint subm. to Ap.-J. Jenkins, E.B. and Heloy, D.A, 197*1, Ap.J.Letters, to be published. Kate, T., 1975, preprint subm. to Ap.J. Stevens, J.C., Rieçler, G.R., and Garmîre, G.P., 1973, Ap.J., 183, 59- Vaîana, G.S-, Krieger: A.S., and Timothy, A.F., 1973, preprint to be published in Solar Physics- Fi gu ri 1A. Emission spectrum of a hot thi n plas- i •chan i snis 1.6 x 10 K with a -cmperaturc oT 1 .6x10 '•'• . The- considered arc brenibst ralilunçi, radial .'£.' recombination and electron col 1 ï s ï on - induced 1i ne emi ss i on. So'or coronal e leneni ibjnddnce 90 10! A (») and a resolution of 0.5 A i "> assumed Kato, 1975). _j 1 , I 1 ' I I 1 J— 10 20 30 40 50 tO 70-10 90 WO A W Figure 1B. As figure "iA. but for a temperature of 5x1D6K (Kato, 1975). i i i - T. 16 ;06*K - - ! - !x. . ' • i - " / ""'^C:'' j' / V ,,-' . / - % • 'H - / '•y I J I E a 16 20 24 28 32 36 40 52 56 60 WAVELENGTH (û) T. SX 10»-K OBSj.1*™ P - y fi A' t 1 ! i J 1! I i . . . 1 ':-: ' WAVELENGTH (Â) Figure 2. Plasma spectra of 1.6x10 and 5x10 K filtered through a 0.85 pm thick aluminized paralyne filter. At 1.6x10 K a significant fraction of the transmitted energy is in 0 VI I lines. At 5x10 K the most significant contribution is from Fe XVII (From Vaiana et al., 1973). Lunnr Occultation of l'oint X-ray ronn-es. C, Rftppin Max-Planck-Institut £2r Fxtratcrrcst rir.riio i- 8046 Garchir.n In X-rny astronomy the accurate localisation of -X-ray sources is of great importance for an identification with objects ob servée": at other wavelengths. In case of a positive identification the measurements performed in the optical, radio or infrared range can be used to learn nore about the physics of the »ray source. Depending on the type c f source one can for example derive information about th>> enission iacchanisr., the n:nss of the stellar objects in binary systems, the presence of electrons and mar;r.etic fields anil a series of other physical parameters. V.'ith the UkURU satellite and rocket experiments it was possible to noasurc the location of X-ray sources to a few arc minutes. The observations of other satellites as UK 5, SAS C and ANS will give an accuracy of 0.5 - 5 arc minutes for 200 - 300 sources. Since the star density in the crp.lactic piar.u reaches 30 per x square arc ir.inutc it is obvious that an accuracy in the arc second range is often necessary for an exact identification. With X -ray telescopes which are the best instruments with re spect to angular resolution about 1 arc second is achievable up to energies of some keV ( HEA0 B )- Comparable resolutions are available at optical or radio wavelengths. Since the lunar occultation method yields the same resolution as a telescope for the localisation of a point source and is not limited to a cer tain energy range, it will be very important for the observa tions v.'ith 2X0SAT, Fig. 1 shows the principle of a lunar occultation observation. To got the position of a point like source the optimum occul tation occurs when the source intersects the lunar rim at an " m £ 21 angle of 45 . During the disappearence and again during the reappearence the counting rate in the X-ray detector changes. and one can find the corresponding tiir.es. Thon the location of the source is given by the position of the satellite and the lunar rim for the times of disappearence and reappearcncc. It is obvious that errors in the position of the spacecraft and the lunar rin will limit the positional accuracy. Further more the structure of the lunar riin introduces an uncertainty. For EXOSAT these positional uncertainties will lead to an error box of 1 x 1 arc second, corresponding to about Ikn error for the position of the spacecraft and the noon. Addi tionally one needs a certain integration time which depends on the intensity of the source to get a statistically signi ficant "hange in the counting rate and for this the following relation holds: (1) I A T ^>KV (I + 2B) A>)T ' where I is the intensity of the source in photons/cm sec (2 - 20 KeV), B the background flux, \ the detection efficiency, A the detector area and T the integration time. R is the sum of three components, the diffuse X-ray background, the instru mental background and a contribution produced via hremsstrah- lung by interplanetary electrons in the lunar surface. The maxirrum expected 'electron fluxes would not significantly de grade the performance of EXOSAT. Fluxes of 10 - 10" electrons/ 2 cm secster above 40 keV would become non negligible, but have only been very rarely observed near the orbit of the moon. Fig. 2 and 3 show the achievable positional accuracy as a func tion of the source intensity. The minimal integration time T calculated with formula (1) is related to the angular resolution by A g = T/2 for the apogee, because the velocity of the moon is about 0.5"/sec as seen from the spacecraft. For the medium energy detector two values for the internal back ground have been used and for the low energy detector the inter stellar attenuation for two galactic latitudes has been taken into account. The flux values at the abscissa of Fig. 3 are the integrals between 2 and 20 keV for a source with an E spec- f44 trum which has been extrapolated to lower energies. For strong sources not the statistics but the positional accuracy of the satellite and the moon's rim are the limiting factors. After this discussion of the positional accuracy it is of im portance which sources can be occulted for a life-time of one year. Since the orbit of the moon is close to the ecliptic plane occultation observations from rockets or satellites with close to earth orbits are only possible for sources lying within about 5° of the ecliptic plane. To cover a larger region of the sky the EXOSAT orbit will be perpendicular to the orbit of the moon and have apogee of 200.000 kn. The apogee can be in the north or the south and the resulting occultation strips are shown in fig. 4 and 5. In both cases roughly 20 % of all kncwn X-ray sources can be occulted. (The figures 2-5 have been taken from the report of the mission definition study group for EXOSAT, Î1S-347) . Dieappearence Reappearence Error box for the position of the source Pig. 1 I I M llll| 1 1 I Ulll| IIM Ull| -I 1 M III;] ^Nigh instrumental background -low instrumental background S/C limiting positional accuracy ' Statistics lim'teu Positional Accuracy and Upper Liwit on size typical galactic - source Medium Energy Experiment l[ ,l ' ' • "" i ' ' ' ' ' '"' ' ' ' ' ' " ' i i i >MI 10 ' 10u Photons/cm'.s [2-20 keV) ?ig. 2 1 I TTTTTT] 1 1 1 I I 1 I 11 1 1 I I I ll-j b" • 15" « <£„ • 0.«5) Range of performances with 15° < b" , 90° b" • 90° and full Moon background (£. • 0-28) Range of perforœances with 15° Légende ; x no attenuation (0.1-2 IceV) Q with attenuation (0.3-2 keV) full moon background —— no moon background Low Energy Experiment 1 • ' ' "" 10w Photons/cmZ.s (2-20 keV) Pig. 3 OCCULTATION STRIP OF A 'lORTHERN ORBIT 3 25 O 50 150 200 250 300 350 Fis. H Galactic longitude OCCULTATION STRIP OF A SOUTHER» ORBIT 0 50 150 200 250 300 350 F|< S Galactic longitude OrTTCAI. TIISI .'IFICATION OF Z-HAÏ S0UHCE3 by H. V. Periston (Rf>yai jreer.wicr. Observatory, Haiisharo, Sussex, U.K.) A .clance at Tabl e I, which lists the size of error box at the date of X—ray source identification (or in one case misidentificationl), shows that the process of optical identification has not to date relied primarily on the accuracy of - e X-ray position. Rather the identifications have been made with peculiar objects near the X—ray position, so that the more peculiar the object the larger the error box can be. The history of **adio source identification shows similarly that peculiar objects which are nebulous or show ultraviolet excess may be identified with positions accurate to about - 30 arc seconds (Wyndham 1966) but that the neutral coloured high redshift quasars needed positions accurate to — 2 to 3 arc seconds (Gent et al_ 1973)- We may only expect to find unexpected X-ray identifications when positions to similar accuracy are available when all positional coincidences can be investigated oy slit spectroscopy. AT IDENTIFICATION 1 4 arc min^ Sandage e_t al (1966) u.v. excess 1 arc min2 (X-ray) Kristian et al (1971) Cyg y-~ 1 Crab « deg2 Bowyer et al (1964) Supernova remnant iC 773 500 arc min2 3U Catalogue Quasar NGC "4151 360 arc ir.in2 Giacconi et al (1972) Seyfert galaxy M 31 15 deg2 Nearby galaxy 2 Objects too faint GX 5 - 1 200 ar sec No identification to show peculiari ties Let me just indicate for your benefit the various optical procedures necessary to make optical identifications given X-ray positions of varying accuracy. If the eirors are in excess of - 5 arc seconds then the most convenient approach is the transparent overlay method applied to the Palomar or SSO/SRC sky surveys (eg Wyndham 1966). If the errors are better than this but still no better than - 0.5 *rc sec then sky survey points or films may still be used and the candidates measured an /. - y measuring ma-nu.^a against existing positional standards (ie AGK3 or SAG) (eg Argue and Taylor 1974)- If EXOSAT produces positions better than this, then comparable optical accuracy can be achieved by taken plates specially for the purpose and possibly by obtaining special meridian observations (the bright stars move around and any positional system thus becomes worse in time). The bes- accuracy obtainable optically is around - 0.1 arc second (eg Argue e_t_ al_ 1-ill), Optical identifications in the galactic plane and particularly towards tt.e galactic centre are made difficult by the presence of ataorptijn. This amounts to at least 1 magnitude per kiloparsec (probably double this in most dir^ti->ns) and corresponds to within a fact.ir two to a column depth of —3.10 hyurjf-er. atom cm" . Thus a BOIb star at 10 kpc will have V^>19 magnitude. REFERENCES Wyndham, J.D., 1966. Astrophys. £., 144, 459. Sandage, A.R., Osraer, P., Giacconi, R., Gorenstein, P., Gursky, H., Waters, J., Bradt, H., Garmire, G., Steekanton, B.V., Oda, 1Jt., Osawa, K. and Jugahu, J.f 1966. Astrophys. J., 14J, 316. Kristian, J., Brucato, R.r Visyanathan, N., banning, H. and Sandage, A., 1971. Astrophys. J., Letts. I6g, L91. Webster, B.L. and Murdin, P., 1972- Nature, Lond.., 2££, 37. Giacconi, R., Murray, S., Gursky, H., Kellogg, E., Schrier, E. and Tananbaum, H., 1972. Astrophys. J., 1j8, 2fi1. Bowyer, S.t Byrara, E.T,, Chubb, T.A. and Friedmann, H., 1964. Nature, Lond., 201, 1307. Argue, A.N. and Taylor, C.M., 1974- The Observatory, 94, 295. Argue, A.N. Tucker, R.H., Yallop, B.D., Kenworthy, CM., Elsmore, B. and Ryle, M., 1973. Mon. Not. R. astr Soc, I64, 27P. Gent, H., Crowther, J.H., Adgie, B.L., Hoskins, D.G., Murdoch, H.S., Hazard, C. and Jauncey, D.L., 1973. Nature, Lond., 24J, 261. Identifications of X-ray sources with radio bjects James LEQUEUX, Dêparcenent d«? Radioastror aciie Observatoire de Meudon, 92190 - Meudon, France ï. Positional capabilities of radiotélescopes. Present radio interferometers reaclt a resoloing power of a few arc seconds. Typically, the positional accuracy is a few per cent of the resolving power when the signal to noise ratio in good, and has reached 0Ï03 in both coordinates (Ryle and Elsmore with the 5-km interferometer at Cambridge). Table 1 lists the most interesting equipment available at the time of the launch of EXOSAT. Host of the interesting instruments lie in the Northern hemisphere ; the interferometers in the Southern hemisphere have only a 1* resolution, and no new instrument will be built there before the 1980's. A I" positional accuracy in right ascension is easily reached even for very faint sources for - 45° < 6 < * 90°. In declination, mapping and comparable accuracy in position can be obtained for 6 > 10°. Mapping is difficult but not impossible at lower declinations, and the positional accuracy is then lower ; however a sufficient accuracy in 6 is still possible at 6 > - 40° if the field is not too crowded and for not too faint sources. The Bonn 100-m dish at high frequencies is one of the most interesting instruments for this purpose, and has a very high sensitivity. Very long baseline observations currently give 1" and should in the future reach a much better positional accuracy, even for ô = - 30°; however their use is restricted to compact objects with flux densities probably ~ 0.2 Jy at contimeter wavelengths. we now turn to the scientific programmes. II. Galactic astronomy The synthesis radio telescopes have the advantage of rapidly surveying crowded fields and locating faint and even variable radiosources with an accuracy similar or better than attainable with the X-ray occultation technique. Confusion is usually not a problem for this purpose. Many X-ray stars (binaries etc..) have proven to be faint radiosources, and often lie in crowded or obscured optical fields. Identification of X ray stars with radio objects will help finding their possible optical counterpart, and will be a major field of activity with EXOSAT. Filamentary structure in supernova remnants can also be napped with high accuracy in radio (see e.g. the maps of the Tycho SN'R) . A comparison with similar maps in X rays will prove to be very fruitful. Howerver not many interesting SNR are likely to be occulted in the EXOSAT mission . The galactic center is a region of very high interest. In X-ray, absorption prevents its study with a low-energy imaging device, thus EXOSAT could provide a unique opportunity for a high-resolution study using lunar occultations. Optical observations are impossible, but detailed maps in the infrared and in radio with a relatively good resolution and positional accuracy have recently been obtained. Several objects are of particular interest and deserve a coorûinaLea A-ray - idciiu - iK scuuy ; j s AH at. less than 1° from the nucleus, the radio - IR nucleus itself (which may contain a supermassive star) and another strong 2.2 p source close-by which could br another supermassive star. More progresses are expected in the future in the radio observations. III. Extragalactic astronomy. Most extragalactic X-ray and radiosources can be observed optically, and their positions can be measured with a better than 1" accuracy, either optically or in radio (however excellent positional accuracy is more diffi cult to achieve optically for nuclei of galaxies because of the surrounding light). Whether radio or optical measurements are to be used for identification is a matter of convenience ; however it should be kept in mind that optical observations are linked to the fundamental star catalogues and may have systematic errors up to 0'.'3, and that radio observations can be directly linked to an inertial frame of reference through the use of distant quasars as primary standards. Extragalactic radiosources often have acoroplex structure. Most radiogalaxies with the classical double (or more complex) extended structure also exhibit a compact component coïncident with the nucleus of the optical galaxy (ex : M87, and more recently discovered Cen A, Cyg A, etc.). Positional accuracy for all this structure is usually not a problem if a high-resolution radio mapping can be achieved (this is even possible for low 6 sources if they are strong enough) ; this will allow a direct caparison between X-ray and radio structures (ex : the jets of 3C 273 and M87, NGC 1275 (tail radiogalaxies, etc..) ; unfortunately, for most objects the S/S ratio in EXOSAT will not allow a study of the finer structure at scales al" seen by long-baseline conventional interfero meters or VLBI. IV. Conclusion A large amount of information of astrophysical interest (e.g. on the emission processes : inverse Compton or other, and physical conditions in the source) can be gained from an intercomparison of X-ray and radio structure of extragalactic sources an obvious goal of EXOSAT. However the same type of study will be very rewarding in galactic astronomy as well, especially for the galactic center where much progress has been made recently in the IB and radio observations, and for which EXOSAT in the occultation mode would be an unique tool. Table 1 : High - resolution radiotélescopes in 1979 Instrument Frequency EW Resol. 6 -range N-S resol. ô-range Remarks Cambridge 5 km 5 GHz 2" > - 25° 2'/|sin 6| > + 10 Positions can be measured in 6 down (for synthesis) to - 30° with sufficient accuracy however fl=0 is a forbidden zone. 15 GHz > - 25° -l"/|sin 6| (fo? synthesis) .. . ""r "solution probably £ limited by the atmosphere Westerbork with 1.A GHz > - 30° !2"/|sin o| extension more sensitive than Cambridge 5 GHz > - 30° 3"/|sin 6| Cal Tech 5 GHz > - 45° 40"/ cos (X-6) 40 N-S baseline exists. Observations can ,_e> be combined with those at Cambridge 10 GHz 10" - 45° 20"/ cos (A-6) > or Westerbork for mapping at 6 < 10° NRAO 10 GHz 3" > - 45° -3"/1s in 6 > + 10° Oblique baseline. Accurate positions for synthesi.) possible at 6 Bonn 100 m <30 GHz > - 30° > 25" > 30° Positional accuracy better than V, High sensitivity VLA <22 GHz > - 45° ï I"/ cos (X-c) > - 40° Possibly in partial operation in 1979. NS baseline tun French-German <150 GHz > - 30° S 1"/ cos (A-6) > - 30° Beginning operation - 1981-82 project NS baseline > O'.'OOl > - 40° Present positional accuracy 0'.1 - [" in both coordinates. Southern sources accessible for 5 > -40° instruments in the Southern hemisphere lack sensitivity or/and resolution amnd Sensitivity limitation 0.2 Jy cannot compete with any of the cited northern instruments. No improvement in this situation is foreseen in the near future. 14(1 Observations rit wavelengths from 2JU~ to 1 mm .... . -^ .... 0." ^.y,^ y0lJyj^s-» tranches of Astronomy. Sky .'.!••.••;;• :.'i'."•"• ;^er. jarried out at few wavelengths and in • • c- !•; •••* -d r.^v-s o*' the sky. Hence we can expect a strong •• • • .* : ->:.- :'"i ' -'rom the correlation of X-ray and infrared * :•.•-* J -:is, leidir.g' to an understanding and development :" : * .• b!"--nchec of astronomy. We must also be prepared for :..!'! .-.a-s, and any programming of a search for infra—red -,,:/1 rrsrts of X-ray sources should not lean too heavily - :. our present knowledge and expectations. Ihe type of relation between X-ray and infra-red -::.iss:on -an take many forms : 1 - "he X-ray source is, cr is companion to, a bright but distnr.t star, optically obscured by interstellar dust iu1 s:.i!2 visible in the near infra-red. \ - .-," X-ray source is embedded in a dust cloud. Absorption of UV T>hotons and very soft X-rays, heats the grains wr.ic.h emit in infrared. . - '.i\- X-ray source is surrounded by a cloud of ionised gas. Infra-red radiation (and radio) from free-free emission. ~ - h--:tii infra-red and X-ray emission from the same synchrotron r-.tdig-f ion frequency spectrum. ' - X-ray emission from Compton scattering of relativistic electrons on infra-red photons. •' - Explosive events producing both X-ray and I.F. transient sources. I. Examples of Identification of X-ray sources and Infra-red Objects. :^.:u 7-1 : ;|-.ic, thte hrirht.es t X-ra 'TIS WP]] i d'-r.t, i !"iod optical and radio counterparts. !h-j -issocia' --d infrared source, interpreted as duf- T,C the same therrr.al bremsstrahlung {J"ree-free') scoctrun as the X-ra.y "mission shows a self-absorption spectrum 1 rom which nas been derived an estimate of the dimensions of the source . Not- 1 hat in less intense (more distant) X-ray sources the infra red emission would fall below the level of detectability. Only if the free-free mechanism responsable for infrared is dissociated from the X-ra.y emission (case 3) is i* likely to be defectah1o. 2 - Cyg X-3 : This source has no optical counterpart, but there is a radio source within the X-ray error box. An infra red source has been found within 2 arc sec of the radio sour;vj '•'he intensity of the infrared source is modulated synchronous! with the 4.ri h period of the X-ray source, thus confirming the identification both of the infra-red object, and of the (2) radio source with Cyg X-3 . Models of this source suggest the presence of a circumstellar cloud due to material ejected during the evolution of the binary system 3 - GX 2+5 : A near infrared source with a faint optical counterpart within the Copernicus error circle of 2 arc ir.in dinmeter has been reported. A positive identification still await s confi rmat i on. 4 - The Galactic Centre : This is a very complex radia and infrared region to which is associated an extended X-ruv source as yet unresolved 5 - Thi? Bxtragalactic )urrps : Extragalactic X-ray sources CBI be roughly divided into two main groups : extended sources wi*h low photoelectric cut-cff, associated with clusters c~ -alaxies, and compact sources with high photoelectric cu- -off associated with active galaxies, strong radio gs'ixies, seyferts and quasars. Infrared emission is found tc be associated with the latter group. The relative lur inosities ir. radio L_, infrared L1n and X-ray emission L n this group show some tendency to correlation, e.g. Lv ^A^ 1 for 3C273, 3C323.1, Cyg A, KGC 1275 NGC 41^1 and Ce Ai but there are notable exceptions. For instance M82, wh -e radio and infrared luminosities are similar to those -2 of ;GC 4151, has an X-ray luminosity l.-^IO L.A , and NGC A TO*, lOf:, another bright radio and I.F. object, has not been ir. Luded in the UHURU catalogue. II" . Possible uses of high precision in the location of the X-ray sources 1. Identification of V e infrared counterpart of distant galactic sources The group of bright X-ray sources within 30° 1c p.tude of the Galactic Centre are suspected of being of a ifferent nature to the general population of the galactic (7) ~~ ' They Vnv** ** high -nh^ + opl ectri r nut-off indicating that the optical counterpart will be extinguished, and maximum \ ( ft-) brightness will be observed at A ^0.25 Ar If these sources have luminous binary partners, v, like eg Cyg X-1, the companion '*'ould be visible in the A' 2-3 similar to the sources in the Galactic bulge- ' " . A Cyg X-3- type, if occur>•:" r-r ' i M IL* r-t-icr. should ae s-;en. ïhe presence or absence of an Lnfran:d ccur.terrart '.-nil be L:i itcfli' a strong indicator cf thr ty: r- cf source prt-s-T.t in this region. 2. SearcVi for peaks o^" X-ray emission in e.i! nci d'~nce with peaks of infrared emission at the -IJL! actio '>;ritr-:. Within the extended X-ray source at the Galactic Centre there is n complex infrared s Iructure, probably du<' to thermal radiation from >~uat heated by a smooth distribution. of M-K stars and groups of young 0 stars. The X-rr-.y emission could be due to inverse C^ompton scattering of relativis+ir electrons on the IR photons. In this case one would expect the X-ray emission to peak in coincidence with the non-thermal radio source at Sg A V/est. If. alternatively, the X-rays are due to thermal emission from unresolved sources, one could expect the X-ray structure to correlate with the 100y**- wavelength structure and shew 3 discrete extended peaks. 3. Location of the X-ray source with respect to the infrared peak in extragalactic sources. Upper limit to the size of the emitting region. In these sources, X-ray emission may occur- either through synchrotron emission, responsable also for the radio emission, or through inverse Compton scattering of the hifh energy electrons on the infrared photens. Alternative models for Cen A. for instance have been proposed, involving both these mechanisms. , The variation in relative luminosity in the three regions among the various sources suggests that one or other mechanism may be dominant in the different objects. The relative site of the X-ray and IR emission, and their v-Z':*"ive -tension will, when available, afford a vital clue to ' >:-- : r.T rrretaiion. û. 3.-;:rcn -'or infrared counterparts of the UXO (unidentified X-r3\ c Sects'). .- large fraction of the high-latitude X-ray sources have not y-t been identified. A search for optical counterparts has led to the conclusion that most of these objects cannot be identif: ?d with QSO or distant rich clusters, and that their X-ray luminosity greatly exceeds that in the optical (12) band . _n the absence of other data on these objects, it seems resorr^ole to search for them in the infrared wavelengths. If tlieir in: -ared luminosity is of the same order of magnitude as or greater than that of the X-ray luminosity, as in bhe radio-galaxit? and quasars, they would be visible in the wavelength ba ,d ^v/#-14/«. at a levels of 10~ Watt/m H . The importance of precise location of these objects lies in this case in the necessity for a prolonged observation with small diaphragm to distinguish such weak sources against the diffuse brightness of the background. 5. Identification and observation of the infrared counterpart of "transient" sources. By "transient" we mean here sources of a limited life-time, but long enough to permit their inclusion in the occultation manouvres. Observation of such sources in all wavelength bands is obviously of the greatest interest. The infrared region presents the particular advantage that in some cases, e.g. novae, ~.he infrared emission from an expanding shell reaches its maximum after the peak luminosity at other wavelengths, thus in rreasing the probability of being able to make use of the precise X-ray location before emission die down. 1 "1 1.>I-:LgpMuer, J.R.Oke, E.Kecklin and G.Garmire, Astrochys.J., 1_55, 1, (19C9) 2) E.E.Becklin, G.Neut'eoauer, F.J. Hawkins, K.D.Mason, P.W.Sani'ord, K.Matthews, ::. G.Wynn-V.'illiams, Nature, 24£, 302 (1973 3) E.P.J. Van Ser Heuvel and ':. De Loore Astron. and Astrophys. ?b_, .387 (1973) 4) I. S. Glass and It. W. Feast nature, 24jj, 39 (1973) 5) E.Kellogr, H.Gursfcy, S.Murray, H.Tananbaun an? H.Ciacocni, Ast.roohys.J. 169, 1-99 (1971) G) G.F.Fieke arid F.Iow AKtrophys.J. V76, L95 (1972) 7' C.Dijworth, L.Marasc1-i and C.Reina Action, and A trophys. 28, 71 (1973) 8) C.Reina and M.Tarenghi Astron. and Astrcphys. 26, 257 (1973) 9) G.W.ClErk, M.I.T. prepr nt CSR-P-75-5 (197b) C.R.Canizares and J.E.N -ighbours, If.I.T. preprint CSR-r-75-7 (1975) 10) W.Ticker, E.Kellogg, H.Gursky, R.Giacconi and H.Tananbaum Astrophys.J. 160, 715 (1973) 11) G.C.Perola and M.Tarcrghi Astronomy and Astrcphys. 25_, 461, (1973) 12) N.Bahcall, Texas Symposium Bnc. 1974 and Astrophys.J. 193, 529 (1974) /•-stronornic^l Institut»* MS 7 i- : • -r.-ir.ly c"r.rcntrnte on the bin-"!"" r.ourres. Our knowlodre of l.^r-r" i^ rt i ! 1 very poor. The only thi iifrp whi eh we know with some mrr.-Mn.ty --re frf. Cîurrky and Grhreirr 1^): T. ••Tf • re Y.-vy binaries, ."-nd r. ^rr-ret ior. of Tetter or.to * compact rt*r ' ne jtrcn star or black hole) is the r.i-jsc of the X--ny emi"i-ioi 'the failure to detect pver. soft X-r^y? from catn.-'y-mic vri-'Hes shown that accreting >-i:.e ^v-rf? cir. be exclude:/. "owevpr. in order to show how little we do in fact know, let us con- .-i.r.r-r the ei^ht b:r.nries thst -.re- preEently known (table 1 ). They can bf- :-viiec into - "t least - four physically different classes of u^-'prtf - ••crordinf: to either the nature of the X-ray source or that of i *,.- ronp.-Tiion.. I f San^forrf* ;• new object (with q 6-75 - minute perio- :: f.i < y) i.c ir.'-l uded, there m-*y :«> even five different classée of objects. T*. ; ; cLe*-r that re.ro-p we can -ir*;w nr.y reliable conclusions on the n->tjre 'ni evolutionary statue of +*ich of these fiasses oT systems, - r.-..ch 1 ,-:r,--er sair.ple of X-rny binaries is reauired. (Tiie only rroup for which we h^ve sample of reasonable size is that of the missive X-ray binaries). /r I will show, one can predict with food confidence thft EXOSAT will be able to discover many more binary sources. !-z to the accretion process itself: here practically nothing is known with T:ny certainty. V*c GO r.ot ev»>n knew the answers to the most simple restions such as: i.-- tr.c rccrr t: or. !--e zo -T-e transfer fror ? companion which overflows its Soche lobe or is it due to the rapture of matter froT r otellar vine of thç companion? Is there or is there not a disk, ?nc what is its structure?. I will now further restrict my tolk to these two points: Ci) accretion, ^ni wh?>t EX03£T is expected to le?rn us about it, and Cii) the number of r.evj X-ray binaries that one expects to be diccovered. 1 ->. The flccretior. rm-f^. ( i ) Roche - lobe overfj owi; stel'..-.r «in-i On theoretic?! 1 nrour.dr one cxnoi^tr cf. V-T. lier. Heuvel 1?'7' a) t:.nt the nbovc mentior.ed two type?" o"" T r tr-'ms f er yi el ri the IH.'PF 'iccretion a Function of the rr-sr !•'^ oT the nan - der;enern tc coananion st^r) . The corresponding 1 i f*-tires ot r.ynteirs in these accretion ctnrcr, nre listed in the thi rd -inn fi î'th col'jnns he "fully developed" st":fe o: fvoche lobe overflc- ir. preceflR'i stfi^e of brjf-inninr Sache - lobe overflow durinf which the m-nr trsr.nfi "tp i.= still < 10" ••' yr. The duration of thin rt.-ire in 1 i s ted i the l.-.rt col umr Theoretical models of the fire ret ion nroces.-; onto corr.p-.ct sturr: PY.OV that X-rny emission is only exnr^ted if S £ '0 M. vr. For S ^10 M '"vr the source will soon becone e-XTinruisheri bv 1 RCcr 5 _Q absorp-ion (cf. van den :!eu"el ir>7^ =:. for det.-iil.c). r'or 10 " <^ Snccr -r. — ^ 'C one expects the X-rr.y source to hive roughly the Eddinrto- "0' yr critical luminosity. (M vhere H is the mass of the compact st.ir. For S < 10"° K„ / accr ^ S , yr , one expects S —• accr X 10- °8 K0 ",'y r (See Davidson and Ostriker '•')'i'' ) . Taking these constraints into account, one observer, from table ?, that lone - lived (>, '0 yr) steady X-ray sources nre only csnerted for conpinion stsrs with masses K ^. ?.1 M 'Roche lobe over flow) sequence stapes). 1 cf. ::• Khin,-: " 0^ - b^l : roc ;]--0 .-ecticm •'). T" '.• *-:'ove c-or.:;i^crMi or." r.rpr; to rrrnict tir:i '• i.f ~ :•-."•; jvr 1. il1.-:-y -?:r'"nr •:?*'• ;c..fr?d ": ;•' '•u'inr vin", .'-rcv'tion. In r=uri; .-. crt-, t'r.f ccTTirr'. ;•! --r ï :• e::r>ectrd *-o be Toi 1 eiffriî in i tr crr-it V y -. rhock - t-i: ir. the ":. irhly punp-sor.ir ( 7 «-'COG krr. nr-'-'l rtfi'.'ir - • i n ri i'c-~. 1 -vi-ipon s^ C'rtriker 1f?*7* ). Ir. th** r.l terr.-i ti VP rr-rr- - of !?onfcr I otf overflov - or.e exrectr. t'nere to be « (aurr. slower rcvinr;) s t r r a n of matter, which spirals out from near the second Lngrnnri"1" point (cf. Bessellet al. 1f"5). Careful --r - systematic observation? of moving absorption dips ( --ir h-ve been observed by UK - 5 for "en X-~, and by U:i'J?"J ir. Her X-l ar.d *U "TOT - *"*) car. yield the densities ?nd velocities ir thenp "tails'" r.Tià will enable us to distinguish between the posrifcili tier of -hoc'.-, or strear.. Such observations are, therefore, of crucial iTpc.rtr.nce. ( ii) t- phenomenon closely related to the accretion process is the occurence of extended lows, which are seen in On X-^ fine SMC X-1 . Esne^i-Ily the one "turn - on" of Cen X-7; -Aie1'. -"•" arm •i--~. by V'lv^U, fcl 1ov:ir-G ^'-n extended lew , shows tr.at the source was apparently "turning" its way through dense surrounding cold (unionized) nptter (Gursky and Schreier '-tyl^,) This indicates that, apparently, the source had been temporarily extinguished by a too large -iccretion rate. This, together with the fact that the sources which exhibit extended lows happen to be the two intrinsically brightest binary A 4 X-ray source? known (L^ 2x10 L and 5x10 \, respectively for Cen X-? and SMCX-1), might indicate that they are in or close to the stage of beginning Soche - lobe overflow. In such a case they would be expected to disappear within the next few centuries. Kowever, detailed studies (including spectral ones) of turn-offs and turn- ons are required in order to examine whether this hypothesis is correct, ^nd to study details of the accretion processes in these systems. fiii! i-isk - structure. In order to check theoretical models of •-•cere ti or. disks, one needs information on: '• tiraorr- ; •<.?. y.orr-iY.le); t) peri on i r- variation.'- in trie sh.-inc o '- the pulses in X-ray ru ! : arr 'r-.r.: the observed dov'rlinr a '" fi'i^T i r. •. '" - d--y r:;;-;!" ir. Hercule? X-i); c) rr.pjd v.-.rVri ; i ty ir. nor. - tir.'-.vy X-ny ro'.ircer ( en-h *r- the -~r.~ V- pulsar) in order to separate v;.ri ability -r.jp to the di.-=k TroT variability due to other IMLSI-.-, 'c'". - . - _. > :.i: Schrcior '°"7^). The number of binary sources expected to be observable by EX03A7 and the HEAP satellites. Many new binary sources nre expected to be detected by KXCS.'T. f-T two reasons, vi:-.. ( i ) Three superpinnt binary sources p>:i Ft withir •' '-r.oc 'iiKtsriCf ( Gyp X-1, ^U 0900 - 40 s-d U' "'00 - *.v, which, hsve L/~ I.r-K'0* L . ^•00 L- nnd I00 Lg,, respect i vel y ; . 7wo o:~ these ire eelirrinr, «hi le also Cyg X-1 shows a small eclipse - like din in the soft part of itr emission. Since these systems are Po^ul «tion I objects, or." exnects objects of this type to be more or less uniformly distributed throurr.- out the Galactic disk. Assuming a disk radius of 1Ii.5 kpc and takinr statistical ^certainties irfo account one experts 60 _*_ 5r> iher.ee: > P5) sources of this type to exist, in the entire Galaxy obt of which some 40 + 28 (hence: ^ 1?) are eclirisir.tf. Due to the lartre r.enn distance to these sources (some '•? kpc - as compared to some ? kpc far the ti.r-'-r mentioned sources) the mean apparent X-ray 1 aminosity of these binaries is expected to be of the order of î per cent of the app.- Cii) From the current picture of the evolutionary history of the massive X-ray binaries as depicted in figure 1 (cf. van den "reuvel and Heise 19??; De l.oore et al. I97d; van den Heuvel 1975 b) one ejects that, before becoming n sunerçinnt wi th a strong stellar wind, the -or. - -ic.-^norntp coTp-ir.ion :f the /-ny .<=nu-m vit- -i Ti.iin-peq'jnnre rt'r •'or .-fvpr'l rri:lior yeTS. Jlr.T ; : *- r';^-r~iinl J-I^CC (h**'oro t'-c r-t:"" :.'--r o-»; r. " r. :i c :i te i ir "-::;•.'•.•• ~nre) " =?ts only rorc - " - 'C yrr, one vy.ryctr the r-u^ti.-r r ~ -._. riv„ ir- ir.-^cr.cncc- ^t-.rr " ^i-t !.-• ve .-• rollipGcri rompr.nion, to : • .-o~f '.'••1 ! t^ .""CO ;.::nes larger thr-n the nurbp:- of tn.ir>ive su per- 3c, 'i 1-~ct FO^P *0C - • CC r-'jch ryz-lv-s -uc-t he present nnonn the r.-si-ivf ^-ir. - sec.uenep 03 .*t.-.rr. within ^ KPC distune». T-.f !;•; :-r»pf-t roscopie observations wit h the Princeton instrument in !!:e "-îrprnicus s-)tp]lite have shown thnt e^rly-type mnin - sequence nt.-i r '.-:ith " — " ^ Mc ric f IGO hnve sor.' stell-ir wind mssH locr.. How ever, the raass - lor-r nter by win1? rom there stars 'jre .it lenst =o?e "C tir.es sn~-ller than thofp cf the e?rly - ty~e suoer^i ^nts -->nd Cf - stnrs. 'Nevertheless, they E-iy still be sufficient to turn •• '-oransct corcpnnion ir-.to ^n X-ray so-jree with a strength of come C to * !_, •' 4x-c ' to 4y- "Therefore - unless rrtpid rotation of the neutron star r-*nd its maf;neto- sr.l.ere inhibit the accretion - one expects thnt sever."» 1 hundreds of we=k X-ray sources pre present among the OB - type main - sequence stars within ? Kpc. Witn EXOSAT and the !!EAC's many of these nay becoxe rietectable. à. Globular cluster sources. rive X-ray sources are known in or ri ose to globular clusters (table 5). Except for 3U 1746+43 the identifications seem quite certain (cf. Clark et al. 1975Î- "11 of the globular cluster sources are definitely variable which seems suggestive of a binary nature. The presence of these 4 or 5 sources already indicates that X-roy sources are some two orders of magnitude more common among globular cluster rtars than generally among stars in the Galactic disk* Namely, if we fl'-susie the number of stars in a globular cluster to he Gome 10 to 10 v'ionr *9'j°), then the presence of 4 or 5 strong (^>.10 9) X-ray sources .-•n:one the 200 globular clusters in our Galaxy indicates that roughly 1 1174, Proc. Pnd ÎA'J Hepion^î Meeting (Trieste). Gursky, H. and Schreier, E. 1^74, Review pa per presented nt the I.:U Symposium on Variable Stsrn, Moscow, /-ufrust ,,)"5. van den Heuvel, E.P.J, and Heise, J. mtp, Nature, Phys. Sc. ?^Q, 6?. van den Heuvel , S.i'.J. "?""3^_, "p.J. (Letters), June rin the Vrerr). r. ".-rtrorhyrirr -T,~- >"> vi Ï - t i or." . ir.i vrrpi t v c :' 5r-.;^r**l r ! rcrr . -r fro" St-ir;." i>;, :-,c PC.). ?« Classes of Binarv X-ray sources. Sources with * la^sive early type companion Cynus X-"1 *U 0900 - 40 }U 17OC - Ï.7 SHC X-1 Centaurus X-? ?. Pulsating sources *,. Cygnus X-1, type (P = 4,8 hours) Cygnus X-5 4. Scorpius X-l tyr^ < P = C.?Sd, solar-type .îcorpius X-1 companion). Cygnus X-2 (?) <"% Ssndford'r. object (near Centrurus X-j) P - £.7 minutes). o' : :.-- -tel" r r -• <*-., t.:- r.r'T --jrriblc ir. Fui "i y iiovel or>r< • "Roc:.r- ' 30c OV":- ' : 0 • 1 '.'• c - •. r - ! •- : r 3 1 '-ijr-'tl Or —• i \\:::u]^" J % yr) (y-) y < ••-- •• V'e ^~. ,. -. ; _? 1.'. •> i 4.0 11' " •" y ' C ' 1 5.0 _7 PO •.Jx-0 ) 5ijr°:-;:i-Tit£ ""CO d eirlier t'r.rn 3l f; 30 '0 ' - 'C • oc 1C"'' Or-CUrr: V) 10 - iC -ay sourfW in ricbul-- rasters ^'•« ^ ustf r 1^ = KGr "'f'-fi d t = 0 , P = 4. 54 d t = 6.17 x I06 yr. , P = 4. 54 ONSET OF FIRST STAGE OF MASS EXCHANGE. t = 6.20 x (06yr. , P = (0-86 END OF FIRST STAGE OF MASS EXCHANGE (BEGIN OF FIRST WOLF - RAYET STAGE ) HELIUM STAR (= WOLF -RAYET STAR) HAS EXPLODED AS A SUPERNOVA. d t = (0.41 x I06 yr. , P= (2.63 THE NORMAL STAR BECOMES A SUPERGIANT; ITS STRONG STELLAR WIND TURNS THE COMPACT STAR INTO A POWERFUL X-RAY ' -Vi-\^ SOURCE . Figured : Evolutionary picture for n massive X-rny binnry. (Liftimes were computed by De Loore et ••*!. '9?4). soma, STRUCT^ VIA TKL , VJ:.7AT:V: TECH::--;: A.C. Fdl :an Institute of Astronomy, Xa-\i-r.\»y P.-r.r!, Can.'.r.: :.•» i:. - L-: : c.:. r.nr- -rerie:. ^ila: •:••1 re: y.uàying ^ r c ::i L--_-.: -•' i-.cr -7i >i7:£'.-; «sod ::' •hose sources -s (su: •-. as Ljr£* sea;' • r.-r.~: -.'. , : i:x -.- = ;iur^-.e;:t j Decoii.' uncertain at trie . aio^- c:.?_ :;ura- at that level per twenty Leans. -..-,: - :: ;•_ r>ei:.g pvsont ai a i\ux level - 10-20% :. ::;•"•:-. a: out ---Jty, indicating that the flux measure- :.- luv- errors, p-rxreedi-g _0%. This represents a - to :.'_;:•; v.^asavr'~^ :.t s, ", on a ^iven scale, 6 • T.^ke^ -r.ir.gs worse and raises the lower li~it above J latio:.s:.:;- betw-.-en 5 and 6^ (where f?e - Qe , îîg heing ill angle) from the »ray source counts. Such counts 'JH'JF'J satellite for |b| > 20 (Fabian, 1975) and may -urci-s per steradiar. with count rates bet we j- sou: res per hear, -"•- " :'.-."•':L is the surface irrigr.tr.es;; of the X-ray background and the Coma '-•.'•-:, :. : :.:.-:t:cT. of scale 6e. It is clear that confusion need not be iz-i-.-.a:.' :-. zia.6ar~~ez.ts of the Coma cluster or. scales > 6' , although C:.V.T. --; .' very long integration times will be needed in order to reduce The -r.ctor. r.vise *o «sufficiently low levels- Fluctuations in the background — cur.at the 10:% Ipvel when S ~ 5 x 13"" ct s"1 (all count rates in UHURU •"' s "J. "c-jt X-ray sources so far observed have a sufficiently high surface brightness that confusion will not prove a problem, on scales greater than a few rcinutfis of arc, although it wi.I be serious on smaller scales. So far a Euclidean Universe has been as.^..,eG, with source counts of index 2.5 tKtrarc-l-jtit.E continuously to low^r count rates. Assuming that evolution is r.ot present for X-ray sources and that some large dispersion in luminosities e:-:is-H, then the source counts should flatten out at least below - 0.1 ct s-1, TLii wiil tend to make £ in equation (1) an underestimate and make confusion nore sorlojjs at higher1 count rates than shown in Figure 1. Sort- ".e-hc-CF, available fcr measuring source structure are (a) telescopes (- -.- - L keV), (b) modulation collimators (- 0.1 - IOC keV) and (c) L.-.. . -lia: :c-. (< C.I - > 1 >3 keV). Kith the first two the integration tine :-. c-rerrrir.ed by the cbserv?r, but occultation times are dictated by the relative motion of the tfoon and satellite. This provides a fundamental lir.ititio:. - :. this technique, although collecting area can of course be 1 :.::re«:••!:;. The ir^giiig telescope is the superior instrument, bu-- is i;evr-r--;y restricted by its useful bandwidth. Strong X-ray absorption is ::•-•;.-.-.- ir. -any cf the compact galactic and extragaiactic sources, and this l-rther restricts the usable range. The -od-ilatior. collimatc-r has s far be or. used t-j locate pcir.t sources ar.d provide -upper limits or. tr.c-ir sizes, r.xter.sic:. of sources tends *c wash out the T.cdulat ior.s, ar.c^ysis of siurce structure will be remejy cor.piex. Occultations have t-ner, successfully applied ir. the optical and racic wjv.j.er.Er: (cf. Hazard, 137]) and analysis of thn essentially strip scar, produced can be converted into a source brightness distribution (e.g. Figure ." Hazard and Sutton, iy?l). Studies performed for EX0SA7 (MS-34"?, Figure 3), suggest- that ir. the 2-20 keV band, structure in Coma cluster type sources (i.e. of surface brightness - Z.IO-^ ct s-1 sr~-, 2-6 keV) will be resolvable down to a few arc minutes. Structure in even brighter sources will be determinable down to a few arc seconds. Confusion should not prove a problem for such sources, at leasr over the 2-6 keV range, but may be serious at other energies (e.g. < ? keV). Several occultations at different angles are necessary to build up a unique model of the structure. The more probable approach will be to compare structures evident from teles-ope observations at energies < 5 keV with occultation profiles in the 2-20 keV range. This should then prove a useful method of searching for energy dependence in structure and emphasises the importance in having imaging telescope observations available. Cluster of galaxies provide an example to illustrate this point. An inverse-Compton origin for the X-ray emission would suggest a reduction in size at higher energies owing to lifetime effects on the electrons. Trie more likely origin is in thermal Eremsstrahl*jng of hot intercluster gas and size measurements as a function of energy (ar.d in a way temperature! should restrict the means available to heat that gas. Individual galaxies embedded in the general cluster emission, such as NGC 1275 in the Perseus cluster (Fabian et al, 197u) rnay be well studied abeve 5 keV by the cccultaticr. technique. REFERENCES Fabian, A.C., 1975, MNRAS, to be published. Fabian, A.C., et al. 1974, Ap. J. Lett., lS9_t L59. Hazard, C., 1971, in Highlights of Astronomy, Vol. 2, p. 607. Hazard, C., and Sutton, J., 1971, Astrophys. Lett., 7_> 179- (Ibl>20°) (z-fe KA\I) •©OCd •col «Oi Fig. 1 Source count rate, S, ( UTiuru unit=) for which confusion effects =e will be important in a beam of equivalent solid angleiX-e ( ' assutr.ing photon noise negligible. •vi'rtV/ •/**.•-./•'•.. 1-2 ËÊ 1 X J I I J I 00 41" OO^?™ 00*53™ UNIVERSAL TIME - otJ r"-A°c-Facsimile of the observed occultation curve of 628 !-'.c/sec smoothed with a time constant of 1 second- Horozontal scale: Universal time; 1 division represents 2 minutes. Vertical scale: Flux density in arbitrary u-its. • Optical Posits / / t ; i )/i i i 1 ' \> ; ! • 'i il I i i 4 I i • i I i i i I7h27m48s 40s I7h27m32s RIGHT ASCENSION Fig. 2 Derived strip brightness distribution across Kep-Ler's supernova at a frequency of 628 Mc/sec in position angle 58, The distribution corresponds to that seen using a fan beam with a half-power width of 10 arc sec. The solid circle shows the position of the supernova as estimated by 3aade (19M3) from Kepler's data (corrected to Epoch 1950.0) and the broken circle the error in this position. Nebulosities close to the supernova position (Minkowski 1958) are indicated by the hatched areas- The position of the radio source determined with the interferometer of the California Institute of Technology (Fonalont et al 1964. Wyndhara and Read 1965} and the R.A. measured using the E-VI.arm of the Hills Cross (Sutton 1966) are also shown. The curved lines M to l-i show the position of the Moon's limb as it passes over the source. Horizontal scale: Right ascension: one division represents 1 sec of time. Vertical scale: Inclination: one division represents 1 arc min. • urn in. i • mu i n ' I I II 1 I I I I I I I I I I I I I I ill III! 10" 101 d;min (arc seconds) SPM}<\\. sr^uci;, ;. :•; [,MA:II:. /-RAY : ;. ^CF.S S'.l'^^ A. x iGv:^ f Yj Obsc-t'-.atoi*-*1 c,- MCUCON ItvS Amonçi Ik currently known X-ray sources, about 103 are believed to be çisUciic. t\.si of these tend to fall within one c>f the following groups: co'.i'p^rt sc-i.rc?j ',Scc A'1, type), eclipsing binaries, sources in globulir clusters, pL.l^ai's and supernova remnants. Only the latter have so id\* been •i^u.'*. to oxbibit considerable spatial '•tructure and in all five c a «;•:.• s [:• J T.-Me 1) the X-r?y stiuclure correlates rather well with what is obsc-!-v"J en icidio wavelengths. Supernova remnants (SNR) constitute a well-know.-, class of galactic racin sources characterized by a non-Lhermal spec'.'L."", OH extended shell-type structure and considerable polarization, (see Holtjer 1972 for a review). Our knowledge of SNR has considerably advanced in recent years due to extensive surveys and to high-resolution work with aperture s>nthesis telescopes. An example of the gain in infor mation content achieved at radio wavelengths is shown in the case of IC443. Early work at GOT (Kundu and Velusamy 1969) showed little fine structure but subsequent surveys at 21cm with smaller beamwidths revealed much detail (Mill 1972). The recent 23cm map obtained at Westerbork shov/s almost as nvjcli structure as is visible in optical photographs (Ouin et al. 1974). 1C 443 was first detected by UHURU (Giacconi et al. 1974) and 0S0 7 has confirmed tl>e identification (Winkler and Clark 1974). A considerable impruw.'..-ut needs to be made in X-ray spatial resolution before we can match thev- remarkable maps. One example clearly showing that indeed an increase in X-ray spatial resolution brings an enormous wealth of information is that of the Cygnus Loop. Early detection at low energies by the LRL group (Grader et al. 1970) showed the source to be extended. Subsequent surveys by ASE (Gorenstein et al. 1971), Cal lech (Stevens and Gamrire 1972) and MIT (Rappaport et el. 197-":) v:ith one-dinension?.! focussing telescopes have gradually increased the effective résolution to the point where a meaningful comparison can now be made with optical and radio data. Current SNR rr.ods 1 G prpdict X-ray emission throughout rost of the life of the remnant, during the first 10 years a combination of synchrotron and thermal emission processes may be responsible for the X-rays observed from Cas A and Tycho. For older SNR the observed soft X-rays sre most probably due to thermal emission behind the expanding shock front. High spatial resolution X-ray observations of SNR are needed in order to help us understand the nature of the energy production mechanisms in and behind the shock wave and perhaps help to distinguish between different models. This will of course have to be done in conduction with detailed spectral measurements. For example, detailed calculations (Strata and Ladn 1975) predict energy-dependent structural changes behind the shock which should be easily measurable. Several estimates have been made in the past concerning the observability of other SNR in the X-ray range, based on the available radio data and X-ray luminosities (Hovaisky and Ryter 1971 - 1972). Some of the objer s expected to be detectable have in fact been seen (Lupus Loop, SN 1006 . This confirmation shows that SNR do indeed form a class of galactic -ray sources that merit as much attention as in the radio region. Finally, a very unique nonthermal radio source, the Galactic Center (Sgr A), which has sometimes been linked to SNR, has been detected as an X-ray source by UHURU (Kellogg et al. 1971) and might possess extended structure. Tôble 1 X-RAY OBSERVATION'S OF SUPERNOVA REMNANTS Source Angular Distance Remarks Reference Diameter Crab 316 2.0 kpc Structure Ricker et al. 1975 Cas A 4.3 2.8 " Fabian et al. 1973 Tycho 8.1 5.9 Gorenstein et al. 1970 Puppis A 55 1.8 SonCsource ^arnecki et „. ,973) IC 443 40 2.0 Winkler S Clark 1974 Vela X 200 0.5 Structure Gorenstein et al. 1974 Soft Source Cyg Loop 179 1.0 „ Rappaport et al. 1974 Lupus L. 270 0.6 Soft Source Palmieri et al. 1972 SN 1C06 26 4.0 3C 396.1 60 2.B Schwartz et al. 1972 W 44 31.4 2.2 MSH 15-5. 26 3.2 Giacconi et al. 1974 REFERENCES Duin.R.M.,Strom,R.G.,van der Laan.H. (1374) Supernovae and Supernova Remnants, Cosnœvici (Ed.) D.Reidel Publ. Co. p. 295. Fabian.A.C. Zarnecki,J.C.,Culhane,J.L. (1974) Nature Phys. Sci. 242,18. Giacconi ,n.. .Murray,S., Gursky.H., Kellogg.E., Schreier,E., Matilsky.T. Koch.D., Tananbautn.H. (1974) Ap.J.Suppl. £7, 37. Gorenstein,P., Gursky.H., Kellogg.E.M. , Giacconi ,R. (1970) Ap.J. ,260, 947 ni Gorenstein,P..Harris,E. , Gurikv.fi., Giacconi ,1'.. .Novick.R. , vancen Bout, P. (1971) Science,]_7?, 369. Gorer.stein.P. Harnden.F.R., Tucker,W.H. (1974) SAO/HCO Preprint No.57 Grader,R.J., Hili.R.W., Stcering.J.P. (1970) Ap.J.Lett., 161, L45. Hill,I.C. (1972) H.N. 151, 113- Ilovaisky.S.A. , Ryter.C. (1971) Astr.Ap. 1_5, 224. Ilovaisky.S.A., Ryter.C. (1972) Astr.Ap. Hi, 163. Kellogg,E., Gursky.H., Hurray,S., Tananbaum.H., Siacconi.R. (1971) Ap.J. Lett. L99. Kundu.H.R. , Velusamy.T. (1969) Ap.J., 1_55, 807. Palmieri.T.M., Burginyon.G.A. Hill, R.W., Scudder.J.K., Seward,F.O. (1972) Ap.J., Y!]_, 387. Rappaport.S., Doxsey.R. , Solinger.A., Borken.R.(1974) CSR-P-74-117 Ricker.G.R. , Scheep'aaker.A. , Ryckman.S.G., Ballintine.J.E. , Doty,J.P., Downey,P.M., Lc- in, kl.H.G. (1975) CSR-P-75-4 Schwartz,D.A., Bleach,R.D., Boldt.E.A., HoU.S.S., SerlenritsosJ'.J. (1972) ApJ. Lett. J_73, L51. Stevens.J.C, Garmire.G.P. (1972) Ap.J. Lett. IbO; L19. Straka.W.C, Lada, C.J. (1976) Ap.J., 295, 563. Winkler,P.F., Clark,G.W. (1974) Ap.J.Lett. HU, L97 Holtjer.L. (1972) Ann. Rev. Astr. Ap. 10, 129. Zarnecki.J.C., Culhane.J.L., Fabian.A.C, Rapley.C.G., Silk.R., rarkinson.J.H., Pounds, K.A. (1973) Nat Phys. Sci. 2_43, 4. )} srv~ Hi'- •. 0 : \>\ Kill ,!(•!!: 77 IC 443. Radio maps of Kundu and Vei ,amy (1969) at 6cm, and Hill (1972) at 21cm. mm w idiwr ». MTfll-i- II ••>•- ^U'.vns ,ni,I H.irr.irL- (!•'":• .r.vt K I:-pj;--fI _ t-t ai The Structures of cbctra^aiactic X-ray Sources M. S. Longair Hullard Radio Astronony Observatory, Cavendish Laboratory, Cambridge 1. INTRODUCTION In this "brief survey I will consider the structure of extragalactic X-ray sources from the point of view of the radio astronomer and the theoretical astrophysicist. I wish to make two main points: (a) The radio and X-ray observations of extragalactic objects give complementary information about the physical conditions in these systems. High quality radio observations are therefore invaluable in interpreting the X-ray observations, (b) In interpreting the results from an occultation satellite, one should adopt the approach of testing specific hypotheses concerning the origin of the X-ray emission. For many extragalactic systems detailed predictions of the distribution of X-ray luminosity may now be made on the basis of an improved understanding of the physics of radio sources and of clusters of galaxies. 2. EXTRAGALACriC X-RAY ASTRONOMY FROM THE POINT OF VIEW OF THE RADIO ASTRONOMER Among the major unsolved problems of extragalactic radio astronomy are the origin and evolution of the powerful double radio sources. The typical source, such as Cygnus A (figure 1), consists of a central compact radio component coincident with the nucleus of an active galaxy or quasar and the double source components themselves which are located far beyond the confines of the associated galaxy and are often foiuvi to consist of a compact "head" and trailing structure of lower surface brightness. In Cygnus At the central component is intrinsically weak but in other sources, especially the quasars, it can be as intense- ns the c^ter components. In interpreting the radio observations of such sources, the radio astronomer expects at least > distinct types of X-ray source to be associated with the radio çnlaxy or nuasa*. (i) Compact nuclear X-ray sources. In the nuclei, there must be ultra- 2 2 4 relativistic electrons with Lorenta factors S/- E/n c ov j.0 -10 . i:itw:oc X-ray emission is therefore expected due to inverse Conpton scattering of the nuclear radio etiissior. itself or of mil J inetcr •and far inf' r,d photons which may be present in large quantities in galactic nuclei. It in alpo possible that there could be an observable X-rny cnmpo:jor.t ar. a rr.-jult of brecsatrahlunp from very hot gas clouds heated by vinrent r^Ii-r^es o!" onerr;,- in fhu nucleus. (ii) X-ray eaissian associated with extended radio s true*, m,-. The electron-, in the components of double sources and ir: r-idio halos arr inferred to havo y~10 and therefore they must be rourren of X-ray:, due le Invertie Compter. scattering of the microwave background radiation. Ln practice, only the halos are viable candidates for the origin of the X-ray enission from the vicinity of radio sources because synchrotron losses are probably the dominant energy loss mechanism for the electrons in the double source components. A possible exception iB M82 (see below) which is not a double radio source. In practice, surprisingly few genuine radio halos are observed around intense radio sources. (iii) X-ray enissioti from hot gas ia clusters. All aodels for the evolution of the double components require the presence of intergaloctic gas in the vicinity of the radio source (see e.g. Longair, Ryle and Scheuer 1973). Since the components are located well outside the confines of the associated galaxy or quasar, the gas must be intergaiaotic. Furthermore in many cases the sources 8 are located in clusters of glaxies. Only very hot ga£, T •*- 10 K, can lorre '• stable intergalactic1,atIDOSphere,' in the gravitational potential well of a rich cluster of galaxies - cooler gas must collapse to the centre of the system. Therefore clusters of galaxies are likely to be sources of intense hremsstrah- lung fro- intergalactic gas. (iv) Hot gas around active fialaxies. The energy in the radio source components must eventually be dissipated. It is therefore possible that .e intergalactic gas surrounding active radio galaxies will be heated to a high temperature by a conbination Df plasma instabilities and ionisation losses of the ejected streams of relativistic plasma. There may therefore be thermal bremsstrahlung halos around active galaxies. 3. THE IDENTIFIED EXTRAGALACTIC X-RAY SOPRCES It is interesting to look at the identified extragalactic X-ray sources in the light of these possibilities. 3.1 Ccmpact extragalactic X-ray sources. The recent Copernicus observations of Centaurus A (Davison, Culhane, Mitchell and Fabian 1975) and of 3C390.3 (Charles, Longair and Sanford 1975) are evidence for compact variable extragalactic sources. In each case there is a compact radio source associated with the nucleus of the galaxy. 3C390.3 (figura a) is particularly interesting ia that it is associated with an N-galaxy whose optical emission is also known to be variable. 3-2 X-ray emission from extended radio source components. No example of X-ray emission from the components of double radio sources is yet known. On spectral grounds it is unlikely that the X-ray emission from Cygnus A is associated with the radio components (Longair and Willmore 197'+). One source in which there may be a direct link between the radio and X-ray structures is H82. In this source, Hargrave (197*0 has shown that the radio and intense far infra-red emission originate from the same volume and that inverse Compton scattering of these far infra-red photons by the relativistic electrons responsible for the radio emission can account for the observed X-ray flux from MÔ2. A direct test of this model is that the X-ray source should have the same dimensions as thy infra-red and radio sources - i.e. about 'jQ'' x 15" arc. 3-3 X-ray sources in clusters of flalaxipr;. These sources are among the most important discoveries by the UHUHU satellite. Ï will only consider in this section those luminous X-ray sources (L ^ 2 x 10 erfj s~ ) associated with Abell Richness class 2 clusters (sec Bahcall 197*0. I have included in Tabic 1 the radio source pair 3C129/129.1. Only in the case of the Perseus cluster is there X-ray spectral information which favours the thermal brer.s.strnhlunç interpretation of the observed X-ray emission. From the point of view of the radio astronomer, however, there are two indf -*it pieces of information which favour the thermal bremsstrahl'ing in-> ation for all of these sources. First, in the second rolmnn of Tab. 1 a tick indicates the presence of a radio trail source in the cluster of gf-laxLes. Figure 3 shows a rocent exarple in Abcll **Q1 discovered by Slingo (197**)• The radio trail sources ore intrinsically very weak radio emitters and pose a rather different set of problems from those encountered in classical double radio sources. Their importance is that they are weak radio sources in which the radio emitting material is swept out of the galaxy by the relative motion of the galaxy with respect to the ambient intergalactic gas resulting in a long radio trail attached to the parent galaxy (see e.g. Jaffe and Perola 1973)- 'Hie trail itself must be "bent" by motions in the intergalactic gas. There io clearly a correlation between the presence of such sources and the presence of X-ray emission. Estimates of the particle density in the inter- galactic gas necessary to produce such distorted structures aro in excellent agreement wi.th those derived frcm the thermal bremsstra>.'.ung hypothesis for the origin of the X-ray emission. Table I îhc properties of radio sources associated with intense X-ray sources in Abell clusters of galaxies of richness class 2. These sources have X-ray luminosity ^2 x 10 erg s" (see Bahcall 197*0. ,*belj cluster Radio source Presence of a Presence of a radio trail component with source very steep low frequency spectrum A iOi ")C1J.17 y y *. "26 (Perseus) 3C83.1B/84 y/y ci) y A 79, 3C218 no information A 165f, (Coma) 5Ci y yu) •' 2199 3C33S - - A 2256 iœ 7S.26 •probably (3) y - Cj'EEus fi (4) - - - 3C129/129.1(5) V Notes vl) There are three radio trail sources in the Perseus cluster. U; Steep spectrum radio component associated vâth intergalactic medium Although not noted by Slingo, morphologically NB 78.26 resembles a radio trail and has a steep low frequency spectrum. Ô^Liis * i>« .ongs to a cluster of richness class 2 according to Matthews, Morgan and Schmidt (196^). ^129/129.1 is included because it belongs to a cluster of unknown nennes? Lt lies in an obscured region; it is one of the most striking examples of a radio trail. 'ZhL- steep spectral, component occure towards the end of the radio trail but do-* not result in an overall steep low frequency spectrum for the Second, in the third column of Table 1, a tick indicates the presence of a component of radio emission with a steep low frequency spectrum. The correla tion of such radio sources with Abell clusters, in particular those clustPi-s of richness clasz 2, was analysed by Baldwin and Scott (1973) and the detailed high resolution observations of these sources were made by Slingo (197^,^). Slingo found that the steep low frequency component originates in régions similar in linear dimensions and source morphology to those sources in 4hcll clusters with normal spectra (see Riley 1975). In general it is assumed that steep low frequency spectra imply old age and it is inferred that in these sources confinement of the relativistic electrons has been very effective. This in turn suggests high ambient gas densities. Both of these arguments favour high particle densities in the antergalactic -ediua in these Abell clusters and are consistent with the theiinal brensstrah- lung interpretation of the X-ray emission. 3.^ X-ray sources and active galaxies. In three cases, it appears that the X-ray source is more likely to be associated with one active galaxy rather than with the nearby cluster. This statement is based upon the closer agreement ol the TJHOHU position of the source with the active galaxy than with the dynamical centre of the nearby cluster. These are the radio sources 3C66, 3C26*» and 3C27^ (M&7, Virgo A) (see Northover 1973, 1975; Turland 1975). In the cases of 3C261* and Virgo the active galaxies are cluster members; in the case of 3C66, the nearby Abell cluster is a background system unrelated to 3C66. There is a distinct resemblance between the three radio sources morpho logically (figure h)r In 3C66 and Virgo A, there are radio jets emanating from a compact central component; in 3026^ there is no jet but there is a compact central component. In 3C264 and Virgo A, there are diffuse halos around the radio galaxy; in 3C66 the jet points along the axis of n very diffuse double l source. In 5C?é+ï the spessrun of the hole stecnor.s to^.ràs its outer boundary and in 3C66 the western component has a steep radio spectrun. The origin of the X-ray emission in these sources is not clear; high resolution X-ray studies are obviously inportant in understanding the relation between the X-ray and radio sources» i.. PROGRAMMES FOR HIGH RESOLUTION X-RAY STUDIES The determination of the X-ray structures of the above X-ray sources is obviously of great inportance in understanding the astrophysics involved. Let me mention some other programmes in the extragalactic field of great interest and importance. 4*1 The precise location of the unidentified high latitude sourcea Identifications with extragalactic systems have been restricted to relatively nearby objects because the sizes of the error boxes are still relatively large. With higher positional accuracy it becomes possible to identify X-ray sources with faint objects at much larger distances. The principal candidates for identifications must reisain. Abell clusters, active galaxies and quasars. 4.2 Crude .tructural information Even the simple distinction of point or extended X-ray source is invaluable in deciding the likely origir. of the X-ray emission as is indicated by the list of possibilf ties in Section 2. Sotî. progrances 4,1 and 4.2 are possible for even the faintest sources in the tfKilRfJ catalogue. On the- brighte*- sources, cioro anbitious programmes may be attempted. 4.3 The bremsstrahlung from clusters of galaxies Host of the published cotiels for the; distribution of hot gac in clusters have assumed that appropriate to isothermal gas spheres. These models are applicable if the intergalactic gas has infinite thermal conductivity but this is unlikely to be the case. In the opposite case in which the thermal condu.- tivity is assumed to be very small, the appropriate equation of state is an adiabatic law; physically this corresponds to assuninn th-it when an element i gas is taken from the centre of the cluster to the outside, it expands und i JOJ.O according to the adiabatic law. Such models have been developed by Gull and Northover (1973i 1975) in their studies of the hydrodynamics of clouds of relativistic plasma in clusters of galaxies. The attractive feature of su^;:. models is that they result in bounded atmospheres in the cluster, the gravita tional potential being determined by the distribution of galaxies in the cluster. Hie height to which the potential wall is filled is determined only by the temperature of the gas. The presence of individual galaxies results in additional potential wells in the overall potential distribution (see figure 5). In this approximation, the gas fills these potential wells resulting in enhanced emission in the vicinity of galaxies. Such a picture may account for tiie X-ray source centred on NGC1275 which is embedded in a more* diffuse thermal cluster source. The above example is intended to indicate how theory may help in interpret ing the result of X-ray observations made with an occulting X-ray teiesoope. One would expect to find enhanced X-ray emission in the vicinity of ^.11 the elliptical galaxies in the central core of the cluster. Since the ;.-ray emission measures the depth of the potential well, it is possible to esticate the nasses of the individual galaxies in. the cluster by this technique. Another important possibility is that since the hot gas fille the cluster potential well up to a certain equipotential surface, the X-ray «amission auto matically defines the shape of the mean mass distribution in the ^lu ter. Thin estimate complements studies at optical wavelengths. IM «•.H Association with radio structures As indicated above, except in cases such as ti82t it is unlikely that X-ray emission will be associated with compact radio features outside galactic nuclei because the inferred magnetic field strengths correspond to energy densities much greater than that of the ambient photon energy density. ïherefore the X-ray luminosity due to inverse Compton scattering is expected to be such less than the radio luminosity. However, in halo regions, the magnetic fields may be very ranaiT and then the inverse Compton effect would be important. Thin ma;1 be the origin of the X-ray emission from the active galaxies noted in Section 5.4 above. 4.5 Some cosmological aspects I mention oaljr one interesting possibility associated with cluster X-ray sources. It is interesting that it is the Abell clusters of richness 2 which are intense X-ray emitters and one can argue that this should be ao because they strike a balance between having sufficiently high particle densities to be intense X-ray sources but not so high that all the intergalactic gas cools within 10 years. For the Coma cluster the cooling time is 10 years. Systems which have particle densities ten tines greater than Coma will be 100 times more intense X-ray emitters and their cooling tines will be 10 years. Such systems at 2 ** 1 should be intense enough to be observed by OHOHD" at about 1 UHDHJ count per second. Some of the unidentified sources may indeed be Df this type. A modest increase in sensitivity suggests the possibility of detecting even nore distant clusters at Z ** 3 when the time-scale of the universe itself q becomes ~K> years, den some of those clusters which are foiling at these epochs should be intense X-ray sourcee. 2hes«3 estimates suggest that it will soon become possible to study cluster formation at Z^ 5-4 through high résolu- 18? tion X-ray observations. Indeed, because of uncertainties in the time-scale of formation the above numbers are lower limita to the X-ray luminosity, and some of the unidentified high latitude UHDBU sources may already be of this type. 5. CONCLUDING REMABKS The main thrust of these remarks has been to indicate the way in which the interaction of X-ray observations with those at radio and optical wave lengths leads to improved understanding of the problems facing astrophysicists in all wavebands. It should be noted that the most important radio information has been provided by the Earth-rotation aperture synthesis telescopes and the availability of such instruments should influence the orbit selected for the EXOSAT instrument. Theory provides many guidelines for the types of experiment which are likely to be most successful in solving these problems; these tests are well within the capabilities of the EXOSAT instrument. We emphasise (i) the importance of high resolution X-ray observations to complement existing high resolution radio observations and (ii) the many ways in which even simple experiments with high resolution at X-ray wavelengths can produce results of fundamental significance for astrophysics. References Bahcall, N.A., 197^, Astrophys. J., 193, 529. Baldwin, J.E. and Scott, P.P., 1975, Hon. Not. E. astr. Soc, 165, 259. Charles, P.A., Longair, M.S. and Sanford, P.W., 1975, Mon. Hot. H. astr. Soc, 170, 17P. Davison, P.J.N., Culhane, J.L., Mitchell, B.J. and Fabian, A.C., 1975, Astrophys. J., (in press). 18t. Gull, S.F. and Northover, K.J.E., 1973, Nature, 244, 80. Gull, S.F. and Uorthover, K.J.E., 1975, Non. Not. R. astr. Soc, (in preparation). Hargrave, P.J., 1974, Mon. Not. fi. astr. Soc, 168, 491. Hargrave, P.J. and HcEllin, H.E., 1975, Mon. Not. R. astr. Soc, (in preparation). Hargrave, P.J. and Ryle, H., 1974, Mon. Not. R. astr. Soc, 166, 305. Jaffe, W.J. and Perola, G.C., 1973, Astr. Astrophys., 26, 423. Lcngair, H.S., Ryle, M. and Scheuer, P.A.G., 1973, Hon. Not. R. astr. Soc, 164, 243. longair, M.S.and Willnore, A.P., 1974, Hon. Not. H. astr. Soc, 168, 479. Matthews, T.A., Morgan, W.W. and Schmidt, M., 1964, Astrophys. J., 140, 35. Northover, K.J.E., 1973, Hon. Not. R. astr. Soc, 165. 369. Northover, K.J.E., 1975, Hon. Not. R. astr. Soc, (in preparation). Riley, J.M., 1975, Hon. Not. R. astr. Soc, 170, 53. Slingo, A., 1974a, Hon. Not. R. astr. Soc, 166, 101. Slingo, A., 1974b, Hon. Not. E. astr. Soc, 168, 307. Turland, B.D., 1975, Hon. Not. R. astr. Soc, 170, 281. Willson, H.A.G., 1970, Hon. Not. R. astr. Soc, lgl, 1. 11 11 Figure 1. Die radio structure of Cygnus A at 5 GHz observed by the Cambridge 5 km telescope (Hargrave and Hyle 197*+). For the two main components the contour interval is 10 K. Œhe solid regions in the Np and Sf components reach 51 *nd kl contours respectively. ïhe area surrounding the central component is drawn with contour interval 200 K. The half power beamwidth is 2" x 2" cosecS and is indicated by the shaded ellipse. 3C3W3 5 GHz _n 1 ' 1 ' I ' I i I • I ' I ' J ' I ' Figure 2. The radio structure of 3C390.3 observed with the Cambridge 5 tan telescope (Hargrave and McEHin 1975). The source is a classical double source with an intense central radio component. The optical image of the N-galaxy is indicated by a dotted circle. The detailed radio structures of the outer components are indicated by insets on the map with snaller contour interval. The X-ray variability suggests association of the X-ray emission with the compact central radio source rather than the other hypotheses discussed in Section 2. 1/ »'- I . I •- 48 46* 44 42* A7h wra tf «' 02*" sC Figure 3. The radio structure of the radio trail source kCï.?>.Y?k in Aball k)l at 4o8 and 1^0? MHz (Slingo 197**). The declination scale on this map is compressed by a factor sin & (see scale on each map). In sky coordinates the source la highly elongated in declination, the associated galaay, indicated by a cross, lying at the leading edge of the brigheet radio component. Figure k. îhe radio structures of radio sources associated with active galaxies. Ca) 5C66 from observations with the One-Mile telescope at 2.7 and 5 GHz (Nortfaover 197**); (h) 5C264 from observations with the One-Mile telescope at 2-7 GHz (Northover 1975); (c) Virgo A (H87; yczfi) as observed by the 5 km telescope (Turland 1975). Gravitational Potential Distance > Figure 5. A schematic diagram showing the gravitational potential distribution in a cluster of galaxies. Superimposed on the overall distribution are potential wells due to the presence of individual galaxies. Mot gas forming an adiabatic atmosphere in such a composite potential well will fill the gravitational potential well to a height determined only by the temperature of the gas. X-ray Halos K. Spiegelhauer and J. Trumper Âstrciiorr.isches Institut aer Universitat Tubingen Introduction In 19ô^ Overbeck'- pointed out that the scattering of celestial X-rays on interstellar grains should lead to the formation of halos around X-ray point sources. So far halos have not been observed, but this is readily explained by the crudeness of pre sent day X-ray instrumentation. It appears that the dust induced halos could provide a new means of investigating the so far only poorly known properties of interstellar grains. So several authors have theoretically analysed various features of such halos in order to get information on the dust. Some of the results v.ill be reported here. Halo Characteristics and Grain Properties The observable halo features are listed below. Gross features - Angular diameter - Halo intensity relative to total intensity Detailed features - Angular shape - Temporal variation - Energy dependence We want to discuss briefly each point and illustrate how it depends on the grain properties. The differential cross section for scattering uf X-rays on interstellar grains is strongly forward peaked and most of the intensity is scattered into a cone with half aperture angle E/keV • a/0.1 p. It can be chown that for a single size grain model (SSM) a suit ably defined halo diameter is-also given by the above formula ^' whereas grain size spectra show slightly smaller angular dia meters. So we see that for energies of typical 1 keV and expec ted grain radii of o.1 yu thu diameters lie in the arcraln range. Another important characteristic of the halo is its intensity which can be expressed by the total intensity and the scattering depth *c Typical scattering depths at ] keV range up. to about 1. Hov.ever, too great a scattering depth will not be observable because absorption is of course always present and can become much, greater than the scattering at low energies. A more refined analysis has to take into account that the grain radii have a size spectrum, that the grain density along inc line of sight is not necessarily constant and that tne illumi nating source may show intensity variations. For the time being we assume a constant spatial density and no variations of source intensity, but a grain size spectrum of Greenberg type, i.e. g(a)<*exp(-5(a/a0)^). Figure 1 shows two different ^r.^ular dis tributions computed for the SSVi and the Greer.berg spectrurr.. At the abscissa a dimensionless angular variable x is indicated. Below the abscissa there are three scales which indicate the actual 1 keV angular extensions for three different moan grain sizes. From measurements of the shape, i.e. Gl/d0(ô), vie are able to deduce the effective differential scattering cross sec tion^ which gives very strong evidence as to the chemical composition of the grains. Allowing now for time variations, it is clear that they can not be observed in the halo unless the time scales are of the order of the maximum time delay which a scattered photon can have, or greater. If vie adopt typical scattering angles of arcr.in and _.ource distances of several hundred pc, the detours of the scattered photons range from lighthours to lightdays. Variations of the corresponding time scales should be observable in the halo, but we expect them to be delayed and smeared In time. As an example we have computed the evolution of the halo as a reaction on a steplike variation of central intensity for a model ("Cyg X-1") with the parameters n =10" cm" , d=1,5kpc, gr E=1 keV, Itot=1 Ph/cm sec-keV, Greenberg spectrum, 50 cm„ 2 effec tive area of instrument. The results in fig. 2 are thought to illustrate that the behaviour of the grain species differs in the absolute number of photons, in the different angular shape, and in the time required for the formation of the final halo. Theoretical considerations show, that observations on variable sources yield the differential cross section and tie spatial density along the line of sight^J. But if both density and cross section are known, the mean time delays are directly proportional to the geometrical distance. In favourable cases this fact offers a method of distance determination-7'. Near an absorption edge the scattering cross section shows strong variation with energy. So measurements on either side of the K edges of elements which are expected to be constituents of the grains can reveal the chemical composition . Table 1 gives a summary of the observable halo features and its relation to the grain properties. Observational Aspects In principle halo observations can be performed with the EXOSAT MS instrument in the occultation mode and with the low energy instrument in both the occultation moda and the arbitrary poin ting mode. a) Occultation Mode The KS package performance is limited by the restricted obser vation time of a few hundred, seconds and the high number of background counts. Table 2 presents calculations carried out N N 10 — 12fo r cn for GX 5-1 assuming a ratio EI/ H= ea 6rain species. So measurements should be feasible on the brightest sources situated at great scattering depths. As to the LE package, the background is strongly suppressed by the use of collecting mirrors. However, the absolute number of photons collected during occultation is low. Table 3 shows conditions as computed for a source of 1/10 Cyg X-Î strength at a distance of 1.5 kpc, where we have adopted an effective area of 50 cm2 and photons from o-9 to 1.1 keV. According to table 3 graphite could hardly be detected. b) Arbitrary Pointing Kode If we use an imaging telescope we have the possibility of carry ing out long duration observations as well as repeated obser vations and we are able to perform observations on variable sources. The most severe problem here is the fact that grazing incidence optics possess an intrinsic instrumental halo. It is caused by the wings of the distribution of the scattered X-rays 197 which extend iniu the arcmin range and contain u? to a few percent of the central intensity. Fig, 3 shows a distribution which was measured on a flat sample of Kar.igen in our institute toy Dr. Bràuninger and his co-workers. We have entered these values in fig. 2 ("Instrumental Background") together with the particle and diffuse X-ray background. The graph demonstrates that valuable studies on halo histories can be performed if the observations are conducted in situations in which the scatter ing depth is high. References 1) J.W. Overbeck, ApJ U*l, 86*+ (1965) 2) S. Kayakawa, Progress of Theoretical Physics }£, \ZZk (1970) 3) P.G. Martin, Mon.Not.R.Astr.Soc. ,1^2, 221 (1970) k) H. Spiegelhauer, J, Trlimper, Journal of the British Inter planetary Society, to be published 5) J. Trumper, V. Schonfelder, Astron.&Astrophys. 2^, /+*f5 (1973) 6) P.G. Martin, D.W. Sclama, Astrophys. Letters £, 193 (1970) 118 Table 1: Halo characteristics and corresponding grain properties Halo feature ~ Grain property Halo diameter Mean grain size I Kalo intensity Ig/Itot Scattering depth tr Angular shape dI/dB(0) Effective differential scattering cross section dS/d-Q.ff ft!> Time dependent angular Effect, diff. scatt. cross section shape dI/de(t,S) Spatial density along the line of sight Source distance Energy dependence of Chemical identification halo intensity Itt(E) Element abundances in grains Table 2: Photon numbers as counted by the EXOSAT ME package during occultation of GX 5-1 Single Size Model Greenberg Model Signal Background Signaù Background 2-3 keV 1632 6467 3740 3234 Graphite 3-4 keV 603 4519 1395 2259 4-5 keV 212 3461 491 1730 2-3 key 1%314 4042 26572 2020 Sllicste 3-4 keV 5417 2825 11223 1411 4-5 keV 1923 2163 4176 1081 2-3 keV 10163 2309 17932 H55 Ice 3-4 keV 3879 1614 7800 807 4-5 keV 1330 1236 2935 618 Table 3: Photons/occultation from a source of 1/10 Cyg X-1 strength as registered by the SXOSAT LE instrument Single Size Model Greenberg Spectrum Signal Background Signal Background Graphite 5 6 Silicate 45 10 81 Ice 35 2 57 r^fiure Captions Fig. 1: Eadial distribution of intensity in the halo normalized so that IH= 1. Fig. 2: Halo evolution following a steplike intensity variation of central intensity of "Cyg X-l". Fig. 3: Distribution of intensity in an X-ray beam after re flection on a flat sample of Kanigen. " u Radial Intensity Distribution -—/lu 8 1 t dx' o b o- O i i i r • i L/ i —i—i—i—i— - '-M^J- f 1 \ •"-"" in o o' * 2 o•*• / / gl e Si z O' / / m » Mo d 30 ' ro / a> i i i i 1 1 1 • | | • ••I • f i i i i w> 0) +* C •™ k. !B > •3 <0 o> JÉ c IS •-!•>; c E •a IcS ok_ IS O) - o •o c c S o +* a> 9) (A .a \ o O 8 12 4 8 12 F'g a. Observation angle in arcmi Kanigen W Gold * = 13,3 A 0=1° a = 1,5° 10 a = 2° 10 10" 10 0.0 0.9 1.8 2.7 3.6 4.5 arctnin •*& 3 _J I I 1 1 1_ J l_ PROPOSALS FOR THE EXOSAT OBSERVATION PROGRAMME A!!D CHGICE OF OR2" R.D. Andreaerii EXOSAT Project Scientist Space Science Department, ESTEC INTRODUCTION D;OSAT CEuropean X-ray Observatory Satellite) was included in the Organisation's scientific satellite programme by a decision of the Council early in 1973. At that time the 'observatory' character of the programme had been identified, in that, although only a small number of research groups would be able to contribute to the pro duction of the instrumentation, a very large scientific community, covering many fields in astronomy ana astrophysics, would wish :c be able to utilise the data. It was decided that the EXOSAT experiments should be funded by the ESRO Scientific budget, with ESRO responsible for the payloac" management. Calls for proposals for participation in the experiment development Drogramme and the observation programme were issued at the end of 1973. Following an evaluation of the proposals received for the experiments, involving eight research groups, the Scientific Programme Board, in mid-1974, approved the composition of a baseline payload and the contributing research groups listed in Table 1. Ninety four proposals for the observation programme (Table 2) were received from this first solicitation, involving ninety scientists, in thirty three institutes, in six member states. An advisory group, the Observation Programme Panel (Table 3), has considered the observational aspects of the mission, evaluated and classified the proposals so far received and made a study of the orbit selection. It is noted here that further calls for observation proposals will be issued at the end of this year and one year before launch, which is scheduled for late 1979. CHOICE OF ORBIT 2.1 Occultations To maximise the possibilities for occultation, a highly eccentric orbit must be taken with the line of apsides as closely perpendicular to the Moon's orbital plane as possible. An apogee height of 200.000 km has been selected, and inclin ations of 40° and 80° have been investigated with the argument jf perigee chosen so that the apogee lies in the direction of the Earth's north pole (northern orbit N) or the Earth's south pole (southern orbit S). The occultation strip (- 20% of the celestial sphere) is such that of the sources of the 3rd UHURU catalogue one can occult : 33 soi--.;es from N, 40°, 31 sources from N, 80°, (with 26 sources common to both orbits) and 18 sources from S, 40 , 26 sources frorc S, 80° (with 16 sources common to both orbits). W.i*:-. tfi-- results from UK5, ANS, SAS-C, HEAO-A (and HEAO-B) are .jvailaMe the number of occultable sources from all orbits must be expected To increase significantly. IT. '.order to maximise the recovery of data from the eccentric oriit, yet to use only one station in order to limit investment and running costs, an orbital inclination of 80° should be taken. The iur.ar 1. Tor the occultation strips a minimum observation distance EXOSAT - Earth of 50.000 km has been taken, i.e. exclusion of the van Allen oelts. 2. The X-ray sources are identified according to the 3rd UKURU catalogue. 3. The conversion of UHURU intensities to energ" flux if 1.7 x 13 ll (+ U0% due to different shapes in the energy spectrum and calibration errors) ergs cnT^ sec--*- per UHURU count sec" . 4. For time variable sources, the maximum observed intensity is listed. In the case of non-varying sources the uncertainty in intensity is given. 5. If under "remarks" a special orbit is not mentioned, it means that the corresponding X-ray source can be seen from both orbits. 2.2 Correlated Measurements The Observation Programme Panel addressed itself to the possibilities of correlating data from EXOSAT with that obtained in other wavelength regions by ground based observâtiories in the optical, radio and infra-red fields. Although at the time that EXOSAT is ational, there will be several large optical telescopes in i±_; southern hemisphere, the northern sky will still be the better explored since no souchern sky survey comparable to that of the Palomar Schmidt will have been made. In the radio field there will still be a major unbalance of high resolution radio telescopes between the Northern and Southern hemispheres. The high resolution aperture synthesis telescopes will be primarily located in the northern henisphere. While, in principle, any optical telescope can be used with an infra red photometer, the only dedicated infra-red telescopes are situated in the northern hemisphere and balloon borne observations are and will be concentrated in the northern hemisphere. Consequently the correlation of measurements should yield a better ^rïentïfic return for northern sky objects, i.e. requiring a southern crbit for EXOSAT. The Scientific Merits of the Alternatives The mertls of the various orbits are only evident when considering the occultation mode since during the lifetime of the observatory all regions of the sky are accessible for study in the arbitrary pointing mode. The major characteristic of the northern orbit» i.e. for occulting objects in the southern sky, is that the galactic centre region is occultable. This increases the number of occultable sources and means their everage intensity is higher. Due to high obscuration however it is not clear that identifications will always be possible even given the accurate X-ray source locations. The major characteristic of the southern orbit, i.e. for occulting objects in the northern sky, is that the most interesting and intense extragalactic sources can be occulted. Structural, rather than position measurements will be important and given several occultations the precision can be increased. Thus the choice is between an emphasis on galactic centre sources for the northern orbit and extragalactic objects for the southern orbit; with the northern sky being better studied at other wave lengths . The_Recommendation_of_the Observâtion_Programrae_Panel This is quoted verbatim: "Because the OPP regards th? science of the extragalactic sources as more interesting and timely and because "f doubts as to the possibility of making many optical identifications in the highly crowded and obscured galactic centre regions even with excellent positions, the OPP recommends the southern oroit and recalls that the implementation of the imaging telescope will in this case determine a full, complementary, observations programme. If, however, because of non-scientific reasons, this recommendation cannot be adopted, the OPP feels that the implementation of an imaging telescope should be given priority as against the choice of the southern Drbit". TABLE 1 EXPERIHENT PROPOSALS TOR EXOSAT Group Leader/ Experiment Outline Institute Experiment Officer Low Energy Package Nested parabolic reflectors with Cosmic Ray Working Group, H. v.d. Hulst/J. Diccker turret mounted detectora Leiden Mullard Space Science Lab,, R. Boyd/P. Sanford UC, London Space Research Laboratory, C. de Jager/A. Brinkman Utrecht Mtdiua Energy Variable flat top array of argon University of Leicester K. Pounds/J. Hoffman Package filled proportional counters MPI, Garching K. Pinkau/H. Zimmcrmann University of Tubingen J. Trumper/R. Staubert TABLE 2 STATISTICAL SURVEY OF EXOSAT OBSERVATION PROGRAM PROPOSALS (Scientific Objectives) X-RAY BACKGROUND 6 CLUSTER OF GALAXIES » SUPERNOVAE REMNANTS 9 BINARIES 17 POSITION DETERMINATION 17 COMPACT EXTRAGALACTIC SOURCES 7 GALACTIC CENTRE 3 DUST HALOS AND INTERSTELLAR GRAINS 5 OBJECTIVES NOT INCLUDED IN POINTS 1-8 16 TABLE 3 MEMBERS OF THE 03SERVATI0:! PROGRAMME PANEL Brinkoan Laboratorium voor Ruimte Onderzoek, Utrecht, HL. Debrunner Physikalisches Institut, Univ. of Bern, CH; Fabian Institute of Astronomy, Cambridge, UK. Lequeux Observatoire de Meudon, F. Maccagni Universita degli Studi di Milano, I. Penston Royal Greenwich Observatory, Hailsham, UK. Rocchia CEN Saclay, Gif-sur-Yvette, F. Schônfelder Max Planck Institut, Garching, D. Spaca Laboratorio Astrofisica Spaziale, Frascati, I. Hilloore University of Birmingham, UK. TABLF u ; Occultable source by the moor, from a northern apogee orbit uith an incliriatijn of i>o° and/or 80° source name galact. longit. galact. latit. area of 2-6 keV intensity (3rd UHURU 1 in degree b in degree error box (UHURU counts per identification remarks Cdtalogue) (degree) sec) 0001 - 31 10.9 -79.0 5.10 3.2 t 0.4 0012 - 5 100.0 -66.2 0.23 4.3 T 2.0 N40 0026 - 9 104.9 -71.5 1.20 4.3 Î 1.1 HGC 1957 One? - 23 152.9 -86.0 1.20 2.1 * 0.4 0138 - 1 149.4 -61.4 U..C 6.2+ 1.7 0405 + 10 181.; -29.5 0.52 3.4 + 0.4 Cluster Abell 478? 0431 - 10 205.9 -35.0 4.40 3.0 + 0.3 N80 0440 + ï 190.3 -24.5 0.50 5.6 + 0.9 0527 - 5 208.3 -20.7 1.40 4.2 Ï 0.5 M 42 = Orion 1,'ebula N80 0531 t 21 184.5 - 5.8 0.003 947 J 21 Crab N40 0614 + 9 200.9 - 3.4 0.005 60 0620 t 23 189.0 4.7 6.1 5.0 + 0.5 3C 167 = IC 443 .140 0901 - 9 238.5 23.8 2.60 4.4 + 0.8 Cluster Abell 754 N80 1224 t 2 289.0 64.3 0.14 4.2 + 0.5 3C 273 N40 1237 - 1 298.1 55.3 2.80 1.3 + 0.4 WGC 4428 4433 4487? 1252 - 29 303.9 33.b 0.13 4.5 * 0.3 M 80 1439 - 39 325.3 18.3 4. 00 3.3 t 0.4 N60 1617 - 15 359.1 23.8 0.002 17000 sco y.-i K4 0 1700 - 37 347.7 2.2 0.009 102 Star IB 153919 AT 1702 - 36 349.1 2.8 0.001 715 GX 349 t 2 1702 - 42 343.8 - 1.3 0.016 34.0 t 2.6 1704 - 32 352.8 5.0 0.C5S 14.0 + 1.2 1705 - 44 343.3 - 2.4 0.001 280 1709 - 23 0.5 9.2 0.C07 39 1714 - 33 348.2 r 1.0 0.083 11.6 + 2.2 1727 - 33 354.2 0.1 0.008 "65 1776 - 24 1.0 4.3 0.003 60.3 + 2.4 1728 - 16 8.5 9.Ô G.C01 260 N40 1735 - 28 359.6 -.0 0.04 565 1735 - 44 345.0 0.002 210 + 6 1743 - 29 360.0 - 0.3 0.092 40 t 5 s::s 7-;2 - L3? "~ i 1744 - 26 2.3 0.9 :. *';)i 4 5? 1746 - 37 353.6 - :, , , , 0.018 30.7 -r i.8 G'.ob'i^ir cluster: NGC 6441? 17*5 - 3J ?f ? - 4.'1 0.^14 ~-i'! 1 17SO - 25 5.: O.'-J1. :i27 QV [,-^ I lt'2D - 3j 2.3 - "-. 9- 250 ~'o':- :].::• Lissier :.'GC 5C14 1422 - .17 356.8 -'.1.3 16.0 t l.u 1632 - 2j 10.4 - b.9 0. 16 6.1 + O.'j N40 TABLE 5 : Oceultable sources bv the moon from a southern apogee orbit w: h an inclina :icn of 40° and/or 80° source name ealact. lonfcit. galact. latit. area of 2-6 keV intensify (3rd UHURU )- in degree b in degree error box (UHURU counts per lier. ifination relr.artts catalogue ) (degree)2 sec) 0032 t 2'l lltt. 3 -38.3 18.0 6.8 + l.u SSC 150 ICj 00112 + 32 121.5 -29.8 0.114 7.0 j 0.5 0151 y 36 136.7 -214.2 0.914 2.4 + O.14 Cluster Ab» 1 262? 0316 + 11 150.6 -13.2 0.012 117.14 + 0.6 rcrseus Ciu 1er: Ab-ill ^26 0352 + 30 163.1 -17.1 0.006 20.2 + 0.5 Star X FER7 01430 + 37 1614.14 - 7.0 1.9 6.0 ï 0.9 0'4'l6 + «14 160.5 0.3 0.053 6.2 t 0.5 3C 129 Obïa t 23 189.0 14.7 6.1 5.0 + 0.5 3C 157 ! S30 . lUtu * 11 236.9 73.3 0.13 3.6 Ï 0.3 Cluster Ahejl 1367 1225 t 12 283.6 714.5 0.0"1 21.7 + 0.3 ^57; Virgo Cluster 1231 + 7 290.7 69.3 1.2 6.7 Ï l.u TC 3576 1 1257 t 28 56.3 88.0 0.011 H4.8 + 0.3 Co'.na Cluster,: Abell 1656 S80 1U10 - 3 339.2 53.7 0.114 3.5 + 0.5 NGC 5 506 5507 S40 1551 + 15 27.5 146.3 15.0 2.1 + 0.5 Hercules Cluster SS'J 1623 + 5 19.7 31.6 12.0 2.6 t 0." 1 r>8o 1812 - 12 lfl.O 2.14 0.037 12.1 + 1.2 ] 1813 - Il 16.14 1.3 0.001 560 S<40 1822 - 0 30.0 5.8 0.009 36.6 + 1.7 • iCdi - 3 ^D.'l .'.. j 1*. 1" 6.1 I 1.0 lfi37 + 14 36.1 14.9 0.001 .'~v S8C 1901 t 3 37.1 - l.H 0.0314 37 SB0 1906 +09 143.6 0.7 0.52 7.5 + 1.14 j CP"; 1908 t 0 30.7 - '1.0 0.002 199 1=112 * 07 i>2.6 - 1.7 0.770 21.5 T 1.0 i soo 1915 - 5 31.3 - 8.3 0.12 23 Star 26 f ACL? 1950 + 11 51.3 - 9.3 0.05 1 17.11 J 0.9 350 2131 1- 11 1 65.5 ! -28.1 l.'l ' '1.1 + O.H •iiS = NCC 7CJ97? , S30 23>IÏ> • 26 1 106.0 | -3'4.0 7. 0 | 7.0 + 1.2 Cluster Abelà 2065 1 i_. I i i TABLE 6 : Occultablc Sourcesbythe Earth from a northern apogee orbit with an inclination of 40° source name galact. longit. galact. latit. area of 2-6 keV intensity j (3rd UHURU 1 in degree b in orbit error box (UHURU counts per identifie, tion catalogue ) (degree) sec) 0302 - D7 259i4 -57.2 2.00 3.3 + 0.8 \ 0328 - 52 264.4 -51.3 18.00 1.7 + 0.4 IC 1933, }954? 0510 - 44 250.0 -35.9 18.00 2.0 + 0.5 PIC A ? 0530 - 37 241.6 -31.0 1.6 2.5 + 0.3 ( 0657 - 35 245.7 -13.7 2.20 3.0 + C.9 0705 - 55 265.7 -19.9 1.00 3.2 + 0.4 0021 - 42 260.4 - 3.2 0.022 7.5 t 0.6 PUP A 1 0833 - 45 263.6 - 2.8 0.052 9.1 + 1.0 PS R0833-4J5 AT (Vela X) 0000 - 40 263.1 3.9 0.001 1Ô0 Star HD 77JF.S1 AT 1044 - 30 273.4 24.9 11.00 2.2 i 0.8 Cluster Abe 11 1060 Î i i I O Occultable sources by the Earth from a northern apogee orbit with an inclination of 80 0400 - 59 270.6 -44.5 9.0 3.8 + 0.6 NGC 15337 i 0426 - 63 274.8 -40.0 0.87 2.6 + 0.4 0540 - 69 280.2 -31.4 0.022 19.3 t 1.3 LHC X-l ! 1144 - 74 298.7 -12.7 0.15 4.3 + 0.8 1254 - 69 303.5 - 6.4 0.001 25.5 + 0.6 1320 - 61 306.7 0.6 0.14 5.2 + 1.6 1439 - 39 325.3 18.8 4.00 3.3 1- 0.4 I TABLE 8 : Occultable sources by the Earth from a southern apogee orbit with an inclination! of 40° , „ - 1) source name galact. longit. galact. latit. area of 2-6 keV intensity (3rd UHURU 1 in degree b in orbit error box (UHURU counts per identification catalogue) (degree) sec) ! i 1639 + 10 63.9 11.3 5.3 1.0 + 0.6 Cluiter Abell Î199? (3C 315) 1822 - 00 30.0 5.8 0.009 36.6 + 1.7 1837 + 01 36.1 1.9 0.001 270 i 1906 + 09 13.6 0,6 0.22 7.6 + 1.1 1 1912 + 07 12,5 -1.6 0.77 21.5 ï 1.0 1956 + 35 71.3 3,1 0.001 1175 Cyg X-U Star i,IDE 22 6868 AT 1957 + 10 76.1 5.8 0,23 5.6 + 1.6 Cyg A = 3C 105! i i TABLE 9 : Occultable sources by the Earth from a southern apogee orbit with an inclination of 80 ! 0115 t 63 125.9 1.1 0.01 70 0113 + 61 129.5 -0.6 0.027 7.2 t 0.5 1 0258 * 60 138.2 2.0 0.23 2.9 + 0.2 Î 1025 t 81 113.2 27.9 0.69 2.7 + 0.3 3C 390.3;7 2128 t 81 116.1 21.8 1.1 1.5 + 0.3 1 I I I I I ! figure 1: Occultabla sourcto By the Earth by tho noon and earth from a northern aporcee orhtf" with an • < • ct/tcc CL-CLUSTEF10F GALAXIES {OFTEN nEFERENCLD WITH inclination of 80° o l-Hcl/we AIIELL'S CATALOGUE NUMBER) * 37 •-- '-.c O 121-512 et/ste SN - SUPERNOVA REMNANT 0) > 513 cl/ftic <-2Q* *% >-a0t IN DECLINATION .1, 1RANSCNT EXTENDED • « I ei/stc Cl-CLUSTER OF GALAXIES (OFTEN REFERENCED WITH rigure 2: Occultable source! O B-32ct/i«C ABELL'S CATALOGUE NUMBER) by the rcoon and earth from a O 32-U«ct/s«c southern apogee orbit with an O IÏ8-512 cl/stc SN - SUPERNOVA REMNANT inclination of 80° Q > 512 et/iec .1. TRANSIENT «•20*^ >-20" IN DECLINATION ' ï1* EXTENDEO