X-ray spectroscopy of accreting black holes

Abstract

Measuring black hole spin has become a key topic in astrophysics, and recent focus on the spin powering of jets in X-ray binaries has heightened the need for accurate measurements of spin. However, the effects of spin are subtle, and are only imprinted on emission from very close to the black hole. This is revealed since the black hole spin defines the radius of the last stable orbit of the accretion disc, which is smaller for larger spin. Recent advances in X-ray observatories and spectroscopic techniques have enabled spin estimates for a number of black holes in X-ray binaries,but the accuracy of these methods, and the link between spin and jet power,have become very controversial subjects. In this thesis I address the former of these problems through X-ray reflection, which is one of two leading X-ray spectroscopic methods to measure black hole spin (the other being the ‘continuum’ method).

Firstly, I investigate the systematic uncertainties associated with the X-ray reflection technique, and display how model degeneracies can severely affect the determination of spin. After establishing these potential flaws I then performed a systematic study of X-ray reflection during four hard state observations of the black hole GX 339?4, and show that the relativistic effects vary significantly over two orders of magnitude in luminosity. I show that this requires the accretion disc to be substantially truncated from the last stable orbit that is used to measure spin, thus rendering spin estimates impossible in the hard state. Following this I analyse over 500 archival observations of the same source with the Rossi Timing X-ray Explorer. Whilst these data cannot directly measure the inner disc radius, they allow a quantitative investigation of how X-ray reflection and the power-law co-evolve. Since the latter gives rise to the former, this allows changes in the accretion geometry to be revealed, which I show to be consistent with a truncated accretion disc in the hard state, and a gradual collapse of the corona in the soft state. Finally, I present three recent observations of GX 339?4 in the hard state with XMM-Newton, which allow an unprecedented simultaneous constraint on the inner accretion disc radius via the reflection and continuum methods. The two techniques agree, and present further compelling evidence for accretion disc truncation in the hard state.

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