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Institut für Astronomie und Astrophysik

Abteilung Astronomie

Sand 1, D-72076 Tübingen, Germany
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Dissertation Jörn Wilms

X-Rays From Galactic Black Holes: Theory and Observation

The main subjects of this thesis are the theoretical foundations for the production of X-ray and gamma-ray radiation in a hot plasma close to a black hole and the comparison of the theoretical results with broad-band X-ray observations of the spectra of galactic black hole candidates.

Chapter 1 introduces black holes as astrophysical objects and describes the observational basis for the existence of compact objects with masses above 5 solar masses. Since this mass is above the maximum possible mass of neutron stars, the existence of black holes in our galaxy seems very probable.

After this introductory chapter, the physics of the production of high energy radiation in accretion disk coronae is discussed in detail (chapter 2). I introduce the microphysical processes that are important in coronae, i.e., Compton scattering, photon-photon pair production, and reflection of the hard radiation off colder material. The radiative transfer problem for accretion disk coronae is then solved using a non-linear Monte Carlo code I conclude that the generally assumed geometry for the accretion disk corona, a cold accretion disk sandwiched between two hot coronae, is not able to explain the observed spectra. This is mainly due to the fact that the reprocessing of the hard radiation within the disk is too efficient so that the density of low energy photons within the corona is too large. The result is that the corona is very efficiently Compton cooled and cannot sustain the high temperatures needed for the production of the hard spectra. On the other hand, the sphere+disk geometry, where a spherical corona sits on the inside of a cold accretion disk, is able to reproduce the observed spectral parameters. The last section of the chapter describes theoretical models for the temporal behavior of radiation emerging from a Compton cloud. In the case of the sphere+disk geometry, interesting effects caused by the reprocessing of hard radiation within the disk are described.

The second part of the thesis describes the application of the above physical models to observational data. Chapters 3 and 4 describe the observational methods and the instrumentation needed for this task. After this interlude, Chapter 5 describes the results from an observation of the galactic black hole Cygnus X-1 using the Rossi X-ray Timing Explorer. The analysis of these data shows that the thermal accretion disk models are indeed able to describe the X-ray spectrum from 2 to 150 keV. The optical depth of the spherical corona is tau=2.1 +/- 0.1 and the average coronal temperature is kTe=65.7 +/- 3.3 keV. From the temporal behavior of the source it is possible to put constraints on the size of the Compton corona, which is less than a few 10GM/c2.

The final chapter 6 describes first results of the longest observation of the two bright black holes in the Large Magellanic Cloud (LMC), LMC X-1 and LMC X-3. The spectra of these objects are softer than those of Cyg X-1, which might be a result of the Comptonizing medium being a ``normal'' slab-like corona. These observations represent the first detection of these objects at energies above 20 keV. Apart from the results of the long observations I also discuss first results from a monitoring campaign on the LMC performed in 1997. The X-ray emission of LMC X-3 is strongly variable on a time-scale of 200 d. The cause for this emission is not understood yet and further observations are planned for 1998.

The thesis concludes with an outlook on future work planned.

Online-Publikation: http://nbn-resolving.de/urn:nbn:de:bsz:21-opus-1570

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