Probing black holes with X-rays

Fundamental questions about black holes are investigated based on X- ray and soft gamma-ray spectroscopy. We discuss the possibility of deriving the spin of a black hole by either studying the shape of emission features broadened by gravitational redshift and Doppler effects, or by modeling the thermal emission continuum from the accretion disk. Applying the first method to high quality XMM-Newton data we conclude that low mass X-ray binary GX 339- 4 contains a rapidly spinning black hole, while we highlight complications that will need to be addressed in the future related to modeling the disk emission. One of the complications is the unknown value of the color correction factor, which describes how much the spectral shape deviates from a true blackbody emission due to electron scattering. We test an observational method to derive this factor and produce encouraging results. The method is based on fitting the disk emission with a thermal Comptonization model, and the color correction is obtained as the ratio of the electron temperature and the temperature of the seed photons. Subsequently, we analyze high quality XMM-Newton data of quiescent black hole binaries GRO J1655-40, V404 Cyg, and XTE J1550-564, and conclude that the X-ray emission is dominated by the jet as opposed to accretion flows in case of the first two sources. We cannot arrive to a conclusion for XTE J1550-564 because the spectrum lacks the necessary statistics, due to most of the data getting compromised by strong solar flaring. This means that the quiescent state in those two sources is fundamentally different from the other spectral states. Finally, we look at an INTEGRAL survey of the galactic center region to investigate whether the unbroken powerlaw gamma-ray emission, which is a characteristic of black hole systems in certain states, can be found in neutron stars as well. Our results show that neutron stars also feature high energy emission, but the two sources we studied showed signs of a spectral break at around 100 keV. The work is still ongoing, we will need to analyze the rest of the approximately 60 neutron star and 15 black hole spectra to reach any conclusion. We also analyze follow- up observations of a new INTEGRAL source IGR J17497-2821 by XMM-Newton, and find it to be consistent with other observations that suggest it is a black hole system in its low/hard state.