197Lapj. . .165L. .21G the Astrophysical Journal

197lApJ. . .165L. .21G -4 -4 n-21 The AstrophysicalJournal,165:L27-L35,1971April15 (g) 1971.TheUniversityofChicago.Allrightsreserved.PrintedinU.S.A. X-ray sourcesScoX-l,Cyg X-l,CygX-2,andCasA.ThethirdLetterreports thedis- initial resultsconcerningtheobservationofsources alongthegalacticequator.Thesec- variability ofX-raysources;and,whereverpossible, toperformcoordinatedand/or plex regionswitharesolutionofabout30';todetermine grossspectralfeaturesand not beenreportedpreviously. report onascanofthegalacticplaneinwhichweseetwenty-ninediscretesources,sevenhave range toanultimatesensitivityofabout5X10thefluxCrabNebula.InthisfirstLetterwe and toprovideinformationwhichmightbeuseful fortheplanningoffutureobserva- simultaneous observationsofX-rayobjectswith other observers. for sourcesatthelimitofsensitivity;tostudy structureofextendedsourcesorcom- lengths. InthissensewecanregardUhuruasatrueX-rayobservatory. which NASAplanstolaunchinthenextseveralyears.December12wasseventh Kenya bytheCentroRicercheAerospaziali.Thesatelliteisfirstofaseriessmall satellites, Uhuru.ThesatelliteisdesignedtoconductasurveyoftheX-rayskyin2-20-keVenergy covery ofanewclassextragalactic X-raysources,theSeyfertgalaxiesNGC 4151and ond Letter(Tananbaumetal.1971)reportsimproved measurementoflocationthe tions. Forthispurpose,wehavesubmittedfourLetters. InthisfirstLetter,wedescribe astronomical communitywithourpreliminaryresults, bothfortheirintrinsicinterest collected andthatwillultimatelybecomeavailable, webelieveitisusefultoacquaintthe sources intherange2-20keVtoalimitingsensitivityofabout5X10fluxfrom perigee and3°inclinationwithaperiodof96minutes.Theuselarge-areacounters Kenyan people,theoperatingsatellitehasbeennamedUhuru(Swahilifor“freedom”). the essentialfeaturesofinstrument,reportupon thepresentstatus,andsome cision ofafewsquareminutesarcforstrongsourcesandtenthsdegree specific objectinconjunctionwithgroundobservationsatbothopticalandradiowave- and satelliteexperiments.Inaddition,thecapabilitytosteerspinaxisadesired It isalsothefirstinaseriesofX-rayobservationsincreasedcapabilityandcomplexity into orbiton1970December12fromtheSanMarcolaunchplatformoffcoastof anniversary ofKenyanindependence,andinrecognitionthekindhospitality astronomy satellitessponsoredbytheNationalAeronauticsandSpaceAdministration. the CrabNebula(1.5X10~ergscmsec"-);todeterminesourcelocationswithapre- orientation intheskypermitsustoperformconvenientlyX-raymeasurementsona tive improvementuponthesensitivityandcompletenessachievedbypreviousrocket the longtimeavailableforobservationspermitustostudyX-rayskywithaqualita- © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Although uptonowwehaveanalyzedonlyasmall fractionofthedatathathavebeen The principalobjectivesoftheobservatoryare:tosurveyskyforcosmicX-ray The firstsatelliteentirelydevotedtothestudyofcosmicX-raysourceswasplaced This isthefirstoffourLettersreportingpreliminaryresultsfromNASA’ssmallastronomy Uhuru isinanearlyequatorialcircularorbitofabout560kmapogeeand520 AN X-RAYSCANOFTHEGALACTICPLANEFROMUHURU American ScienceandEngineering,Inc.,Cambridge,Massachusetts02142 R. Giacconi,E.Kellogg,P.Gorenstein, H. Gursky,andTananbaum Received 1971February13 I. INTRODUCTION ABSTRACT L27 197lApJ. . .165L. .21G 2 M87, 3C273,andNGC5128arereportedinthefourthLetter(Kelloggetal.1971). NGC 1275(Gurskyetal.1971).SomeimprovedobservationsoftheX-rayemissionfrom stant rateofonerevolutionper720seconds.Oncommandfromthegroundspinaxis containing theX-raydetectorsandaspectsensors.Thespacecraftspinsatanearlycon- can beorientedtoaparticularlocationonthecelestialspherebymeansofmagnetic L28 torquing. Aneworientationofthespinaxiscan,ingeneral,beachievedafeworbits. X-ray photonsinthe1.7-18-keVrange.Thelowerlimitisdeterminedbyattenuation Figure 2showsthebandofskywhichisscannedduringeachspinforanygiven orientation. 840 cmeffectivearea.Thecountersaresensitivewithmorethan10percentefficiencyto of theberylliumwindowscounterplusathinthermalshroudthatisneededto PROPORTIONAL COUNTER BANKS © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Figure 1illustratesthemajorelementsofexperimentalportionsatellite The X-raydetectorsconsistoftwosetsproportionalcounterseachwithabout # 5°X5 FWHM COLLIMATOR Fig. 1.—Majorelementsofthe Uhuruinstrumentation A SPINCONTROL TELEMETRY SENSORS STAR R. GIACCONIETAL. n. INSTRUMENTATION SENSORS SUN SPIN-AXIS 1/2° X5°FWHM COLLIMATOR POWER CELLS SOLAR Vol. 165 No. 2, 1971 X-RAY SCAN OF GALACTIC PLANE L29

Fig. 2.—Band of the sky swept by the two detectors during one revolution of the satellite. The fields of view indicated are FWHM. The full width of the band of sky scanned, if the offset of detectors and the precise values of the fields of view are taken into account, is 12?7. maintain the temperature stability of the spacecraft. The upper limit is determined by the transmission properties of the filling gas. Pulse-shape discrimination and anticoinci- dence techniques are used to reduce the background due to particles and high-energy photons. Pulse-height analysis in eight channels is used to obtain information on the energy distribution of the incident photons. An in-flight calibration system which utilizes X-rays from a 55Fe radioactive source and fluorescent X-rays from Zr excited by a radioactive 147Pm source is used to monitor the efficiency and gain stability of the counters. The two sets of counters are placed back to back and are collimated to 0?52 X 5?2 and 5?2 X 5?2 (full width at half-maximum), respectively. The center of the fields of view of the two detector banks are displaced from the equatorial plane of the satellite at respective angles of 88?9 and 91?2 from the spin axis. (The full width of the band in the sky covered by the two detectors during each spin is therefore 12?7.) This arrange- ment allows us to evaluate roughly the elevation of isolated sources in a single pass. While the 0?5 detector yields a finer angular resolution, the 5° detector yields high sensitivity for isolated sources. The precise value of the increase in sensitivity depends on the nature and value of the background and on the angular extent of the source. Two identical and independent visible-light star sensors are rigidly mounted to each of the two collimators, as shown in Figure 1. Star images are focused onto an N-shaped slit located in front of a photomultiplier. Traversal of a star results in a triplet of signals whose time sequence is analyzed for two-dimensional aspect information to a typical accuracy of 1'. Two Sun sensors of similar design are also included to provide aspect information in the sunlit portion of the orbit.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 197lApJ. . .165L. .21G -1 -1 orbits duetotelemetryfailure aspreviouslydescribed. initiated on1970December 27butwasinterruptedafteronlyaboutthree incomplete the numberanddistribution ofsourcesalongthegalacticequator.Thisobservation was with theultimateprecisionexceptinnighttime portionsofeachorbit. quirement. Therefore,thedatafromsuccessive spinperiodscannotyetbecombined since theSunisoutsidefieldofview sensors(45°)duetothethermalre- of motionsderivedfromstarsightingstothesunlit portionsoftheorbit,particularly not yetunderstoodinsufficientdetailtopermitus toextrapolatepreciselytheequations uniformities initsrateofrotationplusanutation. The minutefeaturesofthemotionare background. lent detectorswithsimilarrise-time-discrimination systems.Theequatorialorbitresults Engineering. Allresultsobtainedthusfararebasedonlyonthis“quicklook”portionof in alowercosmic-rayenvironmentforthespacecraft whichexplainsinpartthelow background observedatloweraltitudesbyrocket experiments thathavecarriedequiva- counts secineachsetofcounters.Thisisnohigher,andpossiblylower,thanthe 2-7-keV non-X-raybackgroundlevelinducedbycosmic-rayinteractionsisabout5 complete data.Thequalityoftheexperimentaldatahasbeenratherhigh.residual thus providelesssensitivityandprecisionthanwillbeobtainedultimatelywiththe the data,andonlyfromstudyofcountsaccumulatedin2.4-6.9-keVband, Space FlightCenterandtransferredviaatelephonelinktoAmericanScience each orbit. tended coveragewillallowtheeventualrecoveryofatleasthalfdataobtainedin the equatorhavebeenrequestedtorecorddatafromUhuru.Itishopedthatthisex- on thereal-timedatatransmission.Consequently,severalotherreceivingstationsalong observing program. restricted, aconditionwhichhasresultedinconsiderablemodificationtotheoriginal within a30°coneoftheSun.Hencepointingcapabilityspacecrafthasbeen was notunderstood,arecurrencehasthusfarbeenavoidedbymaintainingthespinaxis a lower-temperature(about75°F)conditionwiththespinaxisatapproximately30°to mission occurredon1970December27and1971January23.Onthefirstdate, mental portionofUhuruhasoperatedfaultlesslyuntilnow.Difficultiesindatatrans- January 23.Generallywehavemaintainedagivenorientationofthespinaxisforfull the Sun,telemetrytransmissionwasfullyrestored.Althoughcauseforthisfailure Since atthetimeoffailurespacecraftwasinitsmaximumtemperaturecondition of thespinaxistoSunandisatamaximumwhen90°Sun. telemetry transmissionceased.Thetemperatureofthespacecraftdependsonangle day, andduringthatperiodthesamebandinskyisrepeatedlyscanned.Theexperi- spacecraft passesovertheprincipalgroundreceivingstationatQuito,Ecuador. view, Sun-sensordataaresampledinstead.Datatelemeteredinrealtimeat1kcand, from thestarsensorsaresampledevery0.048seconds.WhenSuniswithinfieldof (about 85°F),itwasconjecturedthatthiscouldhavecausedthefailure.Uponachieving in addition,awholeorbitofdataisrecordedfordelayedtransmissionat30kcwhilethe 0?5 secthereareabouttwelvesamplesforeveryelementofangularresolution. similar fashionbutoversamplingintervalsof0.384seconds.Analoguevoltagesignals 20 keVaresampledevery0.192seconds.Datafromthe5°detectorobtainedina In addition,thecountsaccumulatedineachofeightchannelspulseheightfrom1.2to accumulated duringsuccessivesamplingintervalsof0.096seconds.Withascanrate L30 R.GIACCONIETAL. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Analysis oftheaspectdatahasshownthat spacecraftexperiencesslightnon- In particular,weattempted earlyinthemissiontoobtainadetailedobservation of About 20percentofthedataforeachdayarerelayeddailyfromQuitotoGoddard The failureofthetaperecorderonJanuary23madeitnecessarytorelyexclusively Figure 3showstheregionsofskythathavebeensurveyedbyUhuruas1971 Data fromthe0?5detectorconsistoftotalcountsinenergyband2.4-6.9keV in. RESULTS 197lApJ. . .165L. .21G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 0 CMr- LO L32 R. GIACCONI ET AL. Vol. 165

Notwithstanding these difficulties, the present results represent the most complete r- observation of the galactic-equator region thus far in this energy region, the total observ- CT) ing time being about 104 seconds. The data obtained by the 0?5 and by the 5° detectors are shown in Figures 3 and 4. Due to the intermittent nature of the telemetry transmission, there are no data during part of the time, and coverage of the galactic equator is not

100

Fig. 4.—Counting rate in the 2.4-6.9-keV range as a function of galactic longitude from the 0?5 detector. The superposition of the several spins has been obtained by using a spin rate derived from the sighting of the strongest X-ray sources. The resulting errors in misalignment have an effect equivalent to broadening the collimator response by about a factor of 2. A selected region is shown with a reduced counting-rate scale (1:5) to show better the ratio of intensity of the very strong sources. Peaks Cl, Dl, Jl, PI, VI, and XI correspond to sources not previously reported.

© American Astronomical Society • Provided by the NASA Astrophysics Data System No. 2, 1971 X-RAY SCAN OF GALACTIC PLANE L33 uniform. Also, because of misalignments in superposition, the widths of the peaks vary from Io to about 2°. Since the fields of view of the detectors are offset, the band observed by the narrow collimator is centered slightly north of the galactic equator, and the one observed by the wide collimator is centered slightly south. Thus, the Crab Nebula is observed only in the 5° detector. With this one scan of the region we cannot determine the elevation of the sources in the field of view, hence the observed intensities cannot be precisely corrected to yield true intensities. The statistically significant peaks detected in the 0?5 detector (Fig. 4) are labeled Al, Bl, Cl, etc., and are listed in Table 1, where their possible correspondence to previously reported sources is given. Similarly, the sources detected in the 5° detector and shown in Figure 5 are labeled A2, B2, etc., and are listed in Table 2. These designa- tions are intended only for illustrative purposes and will be superseded by appropriate nomenclature when the positions of the sources are refined. In some cases where the previously reported sources had large uncertainties in galactic longitudes, the identifica- tions may be incorrect. Since we do not yet have precise information on the galactic latitude of the observed sources, we consider it premature to attempt a discussion of their distribution with re- spect to other galactic features or to attempt to establish a number-luminosity relation. Data of much greater precision and sensitivity are being accumulated at a rapid rate; TABLE 1 Possible Identification of Peaks Detected in 0?5 x 5° (FWHM) Collimator with Known X-Ray Sources

Source Designation Reference Al. Cas A Bl. SN1572 (Tycho’s supernova) Cl. Not reported previously Dl. Not reported previously El. GX 263 +3 FI.. Cen X-2 Gl. Lup XR-1 HI. Nor XR-2 II. . Nor XR-1 Jl.. Not reported previously Kl. GX 3404-0 LI. Seo XR-7 Ml. GX 3494-2 Nl. Seo XR-6 01. GX—2.5 PI. Not reported previously Ql. GX 34-1 Rl. GX 5-1 SI.. GX 94-1 Tl. GX 134-1 Ul. GX 17+2 VI. Not reported previously W1 Ser XR-1 XI. Not reported previously Yl. Cyg X-l Zl. Cyg X-3

References: a. Gorenstein et al. (1970). b. Gursky, Kellogg, and Gorenstein (1968). c. Friedman et al. (1967). d. Bradt et al. (1971). e. Gursky, Gorenstein, and Giacconi (1967). f. Giacconi et al. (1967). g. Cooke and Pounds (1971).

© American Astronomical Society • Provided by the NASA Astrophysics Data System 197lApJ. . .165L. .21G peaks. C2andE2correspondtosourcesnotpreviouslyreported.correspondsClintheprevious figure. tector. RegionsofhighsourcedensityinCentaurusandScorpius-Sagittariusappearasunresolvedbroad © American Astronomical Society Fig. 5.—Countingrateinthe2.4-6.9-keVrangeasafunctionofgalacticlongitudefrom5°de- 10 11I1i^L 0 100 120MO1601802002202402602803003203400 L2. J2. F2. A2. M2. 12. H2 B2. E2. D2 K2. C2. 02 References: f. CookeandPounds(1971). e. Bowyeretal.(1964). b. Gursky,Kellogg,andGorenstein(1968). d. Giacconietal.(1967). c. Friedmaneíoí.'(1967). a. Gorensteinetal.(1970). Source Possible IdentificationofPeaksDetectedin o 5 X5°(FWHM)CollimatorwithKnown Lup XR-1andNorXR-2 VI andW1 Not previouslyreported Not previouslyreported SN1572 (Tycho’ssupernova) Cyg X-2 Cyg X-l Cyg X-3 Crab Nebula Unresolved complexinScorpius- Unresolved complexinCentaurus Cas A GX 263+3 Sagittarius X-Ray Sources Provided bytheNASA Astrophysics DataSystem Designation TABLE 2 Ji INDEGREES (Cl inTable1) See Table1 Reference d d d a f b e c 20 406080 1 L- 197lApJ. . .165L. .21G previous rocketobservations,thatthesetwentyspinsallowustoachieve.Wenotethe high resolutionintheobservationofgalacticX-raysourcesandshowemergence new sourcesevenattheonlyslightlyincreasedlevelofsensitivity,withrespectto in particular,weplantorepeatagalacticscanduringthecomingmonthofMarch,if lar, wenotetheappearanceofonesource,seenonlyin5°detector,nearOrion. been detectedinprevioussurveysoflowerangularresolutionandsensitivity—inparticu- what wehaveobservedsofar,areconfidentthatUhuruwillachieveallstatedobjec- with themeasurementsreportedinfollowingcompanionLetters.Onbasisof we believe,toestablishthequalityofitsperformance,particularlyifconsideredtogether appearance ofsixnewsourcestooweakorincloseproximitytootherhave Tananbaum, H.,Kellogg,E.,Gursky,Murray,S.,Schreier, E.,andGiacconi,R.1971,ibid.,p.L37. Kellogg, E.,Gursky,H.,Leong,C.,Schreier,Tananbaum, H.,andGiacconi,R.1971,ibid.,p.L49. Bowyer, S.,Byram,E.T.,Chubb,T.A.,andFriedman,H.1964,Science,146,912(seealsoOda,M., kins University,whobuiltthespacecraft;andtoourownengineeringstaffatAmerican Friedman, H.,Byram,E.,andChubb,T.1967,Science,156,374. Cooke, B.A.,andPounds,K.L.1971,Nature,229,144. Bradt, H.,Burnett,B.,Mayer,W.,Rappaport,S.,andSchnopper,H.1971,Nature,229,96. in theastronomicalcommunityand,particular,thatofBrunoRossi. and DickGoddard,whobuilttheexperiment. conduct coordinatedobservationsofX-rayobjectswithotherobservers. time toallowuscompletetheall-skysurveywithrequiredsensitivityand tives oftheobservatory,providedonlythatitisoperationalforasufficientperiod Science andEngineering,Inc.,includingWilliamAyer,GerryAustin,StanMickiewicz, the SASProjectScientist,atGoddardSpaceFlightCenter;tomanyotherpeople encouragement; toMarjoryTownsend,theSASProjectManager,andCarlFichtel, Gursky, H.,Kellogg,E.,Leong,C.,Tananbaum,andGiacconi, R.1971,Ap.J.(Letters),165,L43. Gursky, H.,Kellogg,E.,andGorenstein,P.1968,Ap.J.(Letters),. 154,L71. Gursky, H.,Gorenstein,P.,andGiacconi,R.1967,Ap.J.(Letters),150,L75(seealsoMayer,W., Gorenstein, P.,Gursky,H.,Kellogg,E.,andGiacconi,R.1970,Ap.J.,160,947. Giacconi, R.,Gorenstein,P.,Gursky,H.,andWaters,J.R.1967,Ap.(Letters),148,L119. the program;toengineeringstaffofAppliedPhysicsLaboratoriesJohnsHop- Goddard SpaceFlightCenterwhoaidedintheexecutionanddatagatheringphasesof G. Roman,andLeonDondey,ofNASAHeadquarters,fortheircontinuedsupport Uhuru isstilloperationalatthattime.However,thesedataclearlyillustratetheneedfor © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem This preliminaryglimpseofthedatawhicharebeingreturnedfromUhuruissufficient, We wouldliketoexpressourappreciationJohnE.Naugle,JesseMitchell,Nancy This researchwassupportedunderNASAcontractNAS5-11092. Bradt, H.V.,andRappaport,S.1970,Ap.J.[Letters],159,L115). Waters, J.R.1967,Ap.[Letters],148,L5). We wouldalsoliketoacknowledgetheinterestandsupportofmanyourcolleagues Bradt, H.,Garmire,G.,Spade,Sreekantan,B.V.,Gursky,Giacconi,R.,Gorenstein,P.,and X-RAY SCANOFGALACTICPLANEL35 REFERENCES