17 - Polarization of thermal emission from accreting black holes

Summary

Multicolour black-body emission from the accretion disc around the black hole can be polarized on its way through the atmosphere above the accretion disc. We model this effect by assuming Kerr metric for the black hole, a standard thin disc for the accretion flow and Thomson scattering in the atmosphere. We compute the expected polarization degree and the angle as they can be measured for different inclinations of the observer, optical thickness of the atmosphere and different values of the black hole spin. All relativistic effects near a compact centre are taken into account. We also assess the perspectives for the next generation of X-ray polarimeters.

Introduction

We consider polarization originating from a Keplerian, geometrically thin and optically thick accretion disc near a black hole. At each radius the accretion disc emits black body radiation, the temperature of which is given by the Novikov-Thorne expression for the outer part of the standard disc. The thermal photons are scattered in the atmosphere of the disc and thus the observed radiation becomes polarized. We assume multiple Thomson scattering with different optical depths of the disc atmosphere. The effect of hardening of the energy of photons due to scattering is taken into account via the hardening factor that increases the effective temperature.

Once the photons leave the atmosphere the polarization vector can be rotated due to strong gravity of the black hole. The energy of photons is shifted by the gravitational and Doppler effects.