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X-Ray Emission from Supernova Remnants with Particular Reference to the Cygnus Loop'

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X-Ray Emission from Supernova Remnants with Particular Reference to the Cygnus Loop' X-RAY EMISSION FROM SUPERNOVA REMNANTS WITH PARTICULAR REFERENCE TO THECYGNUSLOOP PROEFSCHRIFT TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE WISKUNDE EN NATUURWETENSCHAPPEN AAN DE RIJKSUNIVERSITEIT TE UTRECHT, OP GEZAG VAN DE RECTOR MAGNIFICUS PROF. DR. A. VERHOEFF, VOLGENS BESLUIT VAN HET COLLEGE VAN DECANEN IN HET OPENBAAR TE VERDEDIGEN OP WOENSDAG 27 JUN11979 DES NAMIDDAGS TE 4.15 UUR DOOR EDUARD HENRICUS BARBARA MARIA GRONENSCHILD GEBOREN OP 23 ME11949 TE SCHAESBERG (L) DRUKKERIJ ELINKWIJK BV - UTRECHT PROMOTOR: PROF. DR. C. DEJAGER Dit proefschrift kwam tot stand mede onder leiding van Dr. R. Mewe ter nagedachtenis aan mijn vader aan mijn moeder aan mijn schoonouders aan 11se in het bijzonder Veel dank ben ik verschuldigd aan allen die hebben medegewerkt aan de vol- tooTng van dit proefschrift. In het bijzonder dank ik mijn promotor Prof.Dr. C. de Jager die door zijn aanmoedigende houding en door de vele discussies een belangrijke bijdrage heeft. geleverd. Mijn collega's van het ANS team, John Heise, Bert Brinkman, Hans Schrijver, Ton den Boggende en in het bijzonder Rolf Mewe, dank ik voor de prettige samen- werking en sfeer waarin dit onderzoek gestalte heeft kunnen aannemen. Bovendien richt ik een woord van dank aan Rolf Mewe die door zijn nimmer aflatende op- bouwende kritiek en zijn voortdurende interesse mij steeds weer opnieuw stimu- leerde en door wie ik een groot gedeelte van de kennis van röntgen spectroscopie verwierf. De urenlange discussies met hem hebben een belangrijke stempel gedrukt op dit werk. Ik ben erkentelijk voor de hulp van de leden van de datagroep, o.l.v. Theo Gunsing, die tijdens de operationele fase van de ANS en daarna veel zorg hebben besteed aan de verwerking van de gegevens. Uit deze groep wil ik één persoon lichten, Frans Ammerlaan. Vooral dankzij de vele hulp die ik mocht ontvangen van Frans zijn de soms omvangrijke computer berekeningen nodig voor dit proefschrift tot stand gekomen. Ik prijs mij zeer gelukkig met de vakkundige medewerking van Ursule van der Jagt voor de mooie uitvoering van het typewerk, en van Hans Braun voor de keurige verzorging van het groot aantal figuren. Bovendien ben ik erkentelijk voor de financiële steun door het Leids Kerk- hoven - Bosscha Fonds voor het bezoeken van een zomerschool in Erice op Sicilië. Tenslotte wil ik mijn aanstaande echtgenote 11 se bedanken voor de opoffe- ringen die ze zich heeft moeten getroosten en voor het typen van gedeelten va^ i dit proefschrift. De voorbereidingen tot deze promotie waren voor haar soms 4en ongewilde degradatie. / INHOUD page SUMMARY CHAPTER I OBSERVATIONS OF SUPERNOVAE AND SUPERNOVA REMNANTS; 10 STATE OF THE ART CHAPTER II X-RAY OBSERVATIONS OF THE CYGNUS LOOP BY ANS CHAPTER III EVIDENCE FOR X-RAY EMISSION OF VM 89 CHAPTER IV X-RAY OBSERVATIONS OF SOME RADIO SUPERNOVA REMNANTS BY ANS 99 CHAPTER V CALCULATED X-RADIATION FROM OPTICALLY THIN PLASMAS, III: 121 ABUNDANCE EFFECTS ON CONTINUUM EMISSION CHAPTER VI NON-EQUILIBRIUM EFFECTS ON THE X-RAY EMISSION OF SUPERNOVA 153 REMNANTS SAMENVATTING 219 CURRICULUM VITAE 223 SUMMARY Since the early X-ray observations of cosmic extrasolar objects in 1962 X-ray astronomy has developed in one of the most rewarding branches in space astronomy. Many rocket and satellite instruments have been sent above the Earth's atmosphere, the first nearly complete X-ray survey being made with the UHURU satellite launched in 1970, which has increased the number of known X-ray sources from about 50 to more than 350. With the launch of the Astronomical Netherlands Satellite (ANS) in August, 197** some sources could be studied in more detail and a search for new X-ray sources was set up. This has resulted in the detection of X-ray emission of flare stars, dwarf novae, stellar coronae, X-ray 'bursters', and a new supernova remnant. The successful mission ended in May 1976 while one year later the ANS burnt up in the upper layers of the Earth's atmosphere. This thesis consists of six chapters each describing observational or theo- retical results related to the study of supernova remnants (SNRs). The first chapter gives some background information by reviewing the present status of our knowledge of supernovae and supernova remnants, both from theory and observations. Also the distribution of all known radio, optical, and X-ray SNRs in the Galaxy is shown and a comparison is made. In Chapter II {GronenschiId, 1979b) we discuss in detail the X-ray obser- vations of the well-known X-ray SNR the Cygnus Loop (see cover) carried out with the Utrecht X-ray instruments on board of the ANS. This remnant, also known as the Veil Nebula, was mapped in X-rays by pointing the ANS satellite towards some 150 positions uniformly distributed over a region of k x4 covering the whole optical counterpart. The total observation time accumulated to nearly h hours, resulting in a soft X-ray (0.16- 0.284 keV) map with a resolution of 10' x 12' and showing a shell-like structure of the X-ray emission. In other energy bands the resolution was less, but the maps enabled us to derive a temperature and density structure of the plasma, shock-heated by a blast wave which is expanding in interstellar space since the supernova outburst some 20 000 years ago. Also the amountof interstellar absorption to the source was found to vary across it. To the chapter is added an appendix which presents the method (Algebraic Recon- struction Technique, ART) used in producing the X-ray maps, together with an analysis of the error propagation in this method. A search for X-ray emission of radio SNRs was carried out during the ANS observational period. Only one, W44,was detected. The discovery of this new X-ray emitting SNR is described in Chapter III (GronenschiId et al., 1978). The source was suggested earlier as a candidate for X-ray detection with current instruments, and it is one of the three X-ray SNRs out of 20 which have no op- tical counterpart. Application of a shock-wave model to the X-ray results indi- cates that the SNR is between 3000 and 14 000 years old. For the other radio sources investigated, dealt with in Chapter IV (Gronen- schild, 1979a), merely upper limits to their X-ray luminosities could be set. Assuming that the X-ray emission stems from the interstellar medium (ISM) which is heated to X-ray temperatures through the action of a spherical expanding shock wave the upper limits yielded some parameters of this model like the density of the ISM, age of the SNR, etc., dependent on the distance to the sources. For some of these radio SNRs the density in the surrounding ISM was found to be a factor ^ 5 below the value which is derived by assuming a plane-stratified distribution of the ISM with a scale height of 120 pc. For the analysis of the observed X-ray emission of SNRs thermal spectra like exponential or bremsstrahlungs spectra are used. In recent years however, continuum and line spectra are more and more often used. This stems from the theoretical point of view that the X-ray emission is from a hot, optically thin plasma of a solar-like composition of the elements. The continuous spectrum of such a plasma, discussed in Chapter V (GronenschiId and Mewe, 1978), is due to free-free radiation, free-bound transitions, and two-photon decay processes. In this chapter it is shown that for the assumed set of cosmic abundances mainly oxygen contributes to the continuum which for temperatures between 10 and 10 K Is dominated by free-bound and two-photon decay emission. For all considered elements effective gaunt factors are presented in this chapter. The X-ray line spectrum that emerges from SNRs is in detail described in Chapter VI. It has been suggested long before this that in SNRs the ionization balance may be out of equilibrium, because the ionization time may be as large 1112 10 as 10 - 10 s, a time that should be compared with ages on the order of 10 s (300 years) for the young remnants to 10 s (30 000 years) for the older ones. Until recently no detailed calculations of an X-ray spectrum under these non- equilibrium (NEQ) conditions were carried out. In Chapter VI we present a few models used to compute the NEQ ionization state of the hot plasma in SNRs. The resulting X-ray spectra show a considerable softening compared with the spectrum for which the ionization balance is assumed to be in equilibrium (EQ), especially for wavelengths longer than about 20 A. This softening is partially due to en- hanced two-photon decay radiation by hydrogen- and helium-like oxygen ions which are overabundant with respect to their EQ concentrations. However, the strongly intensified line emission contributes mainly to the derived softening of the X-ray spectrum. For the considered ages (5000-20 000 years) the hydrogen- and helium-like lines in elements with smaller Z-values are much enhanced due to the overabundances of the corresponding ions and the favourable conditions (e.g., temperature) for the line emission functions, which are described in the appendix of this chapter. The results may be an alternative explanation for the excess soft X-ray emission of the observed dual-temperature spectra of young SNRs, like is shown in t ^Ji 1 for Cas A and SN1006. For the former remnant the computed line fluxes at ,hort wavelengths are in reasonable agreement with the fluxes very recently observed with the Einstein Observatory. REFERENCES GronenschiId, E.H.B.M., Mewe, R., Heise, J., den Boggende, A.J.F., Schrijver, J., Brinkman, A.C.: 1978, Astron.
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