Probing Extreme Gravity with X-ray Polarization
In the coming years, new space missions may well be able to measure X-ray polarization at levels of 1% or better in the ~1-10 keV energy band. This region of the spectrum is critical for observing emission from magnetars, X-ray binaries, supernova remnants, and AGN. In particular, we propose that X-ray polarization may be an ideal tool for determining the nature of black hole accretion disks surrounded by hot coronae. Using a Monte Carlo radiation transport code in full general relativity, we
calculate the spectra and polarization features of these black hole systems. At low energies, the signal is dominated by the thermal flux coming directly from the optically thick disk. At higher energy, the thermal seed photons have been inverse-Compton scattered by the corona, often reflecting back off the disk before reaching the observer, giving a distinctive polarization signature. By measuring the degree and angle of this X-ray polarization, we can infer the emission geometry of the accretion flow and in turn may be able to determine the spin of the black hole.