부산대학교 도서관

The regions close to black holes (BHs) have the most extreme gravitational fields; therefore we are able to probe Einstein's theory of gravity by studying how matter accretes onto these objects. The best data we have of black hole accretion comes from X-ray binaries (XRBs) within the Milky Way, which shine brightly in X-rays. The accretion discs are very small and therefore are unresolved in observations, so we have to study the properties of the X-ray point source in great detail to understand the structure of, and processes within, the accretion system. A key observational feature we observe is the reflection of high energy X-ray photons incident onto the accretion disc, which imprint characteristic features onto the observed X-ray spectrum such as the Fe Kα line. Relativistic distortions to the energies and arrival times of the reflected photons enable the mapping of the accretion region. In this thesis I employ advanced techniques to model the reflection spectrum in observations of two BH XRBs, GRS 1915+105 and H1743-322. First, I study quasi-periodic oscillations (QPOs) in the observed X-ray flux from these systems. There are many theories to the origin of these signals, and these theories make different predictions of how the energy spectrum of the source is modulated over the course of a QPO cycle. I use novel Fourier domain timing techniques in order to phase-resolve the energy spectrum of the source over a QPO cycle, to look for these telltale modulation. I constrain modulations in the illumination of the accretion disc by the high energy X-ray photons, adding growing body of evidence that the QPOs have a geometrical origin. I also present pioneering work testing a model able to constrain the mass-distance ratio of a BH,using the spectral shape and observed flux of the reflected X-rays to constrain the size and ionization state of the accretion disc. I performed a proof-of-principle test using observations of H1743-322, which I combined with a prior distance measurement to constrain the mass of the BH in this system for the first time. This demonstration shows how X-ray spectroscopy can be used to make mass measurements of the many BHs within the Milky Way which are too obscured for current optical techniques.