15 - X-ray polarization from black holes in the thermal state

Summary

We present new calculations of X-ray polarization from black hole accretion disks in the thermally dominated state, using a Monte-Carlo ray-tracing code in full general relativity. In contrast to many previously published studies, our approach allows us to include returning radiation that is deflected by the strong-field gravity of the BH and scatters off the disk before reaching a distant observer. Although carrying a relatively small fraction of the total observed flux, the scattered radiation tends to be highly polarized and in a direction perpendicular to the direct radiation. We show how these new features of the polarization spectra may be developed into a powerful tool for measuring black hole spin and probing the gas flow in the innermost disk.

Introduction

A recent flurry of new mission proposals has renewed interest in X-ray polarization from a variety of astrophysical sources, hopefully marking the “coming of age of X-ray polarimetry” in the very near future. The Gravity and Extreme Magnetism SMEX (GEMS) mission, for example, should be able to detect a degree of polarization δ < 1% for a flux of a few mCrab (e.g. and these proceedings). A similar detector for the International X-ray Observatory (IXO) could achieve sensitivity roughly 10× greater (δ <0.1%, Alessandro Brez in these proceedings). In this talk, based on our recent paper, we focus on the polarization signal from accreting stellar-mass black holes (BHs) in the thermal state, which are characterized by a broad-band spectrum peaking around 1 keV.