X-RAY POLARIZATION FROM ACCRETING BLACK HOLES: THE THERMAL STATE

Abstract

We present new calculations of X-ray polarization from black hole (BH)\naccretion disks in the thermally-dominated state, using a Monte-Carlo\nray-tracing code in full general relativity. In contrast to many previously\npublished studies, our approach allows us to include returning radiation that\nis deflected by the strong-field gravity of the BH and scatters off of the disk\nbefore reaching a distant observer. Although carrying a relatively small\nfraction of the total observed flux, the scattered radiation tends to be highly\npolarized and in a direction perpendicular to the direct radiation. For\nmoderately large spin parameters (a/M >~ 0.9), this scattered returning\nradiation dominates the polarization signal at energies above the thermal peak,\ngiving a net rotation in the polarization angle of 90 deg. We show how these\nnew features of the polarization spectra from BHs in the thermal state may be\ndeveloped into a powerful tool for measuring BH spin and probing the gas flow\nin the innermost disk. In addition to determining the emission profile,\npolarization observations can be used to constrain other properties of the\nsystem such as BH mass, inclination, and distance. New instruments currently\nunder development should be able to exploit this tool in the near future.\n