A model for the polarization of the high-energy radiation from accreting black holes: the case of XTE J1118+480

TL;DR

This paper models the polarization of high-energy gamma-ray emission from accreting black holes, suggesting non-thermal corona processes can explain observed polarization without jets, and makes predictions for future observations.

Contribution

It introduces a new model accounting for magnetic field contributions in black hole coronae to explain gamma-ray polarization, applicable to sources like XTE J1118+480.

Findings

The corona+disk magnetic field configuration can produce high polarization levels.

The model explains polarization without requiring relativistic jets.

Predictions for gamma-ray polarization measurements in future outbursts.

Abstract

The high-energy emission ($400$ keV $- 2$ MeV) of Cygnus X-1 --the most well-studied Galactic black hole-- was recently found to be strongly polarized. The origin of this radiation is still unknown. In this work, we suggest that it is the result of non-thermal processes in the hot corona around the accreting compact object, and study the polarization of high-energy radiation expected for black hole binaries. Two contributions to the total magnetic field are taken into account in our study, a small scale random component related to the corona, and an ordered magnetic field associated with the accretion disk. The degree of polarization of gamma-ray emission for this particular geometry is estimated, as well as the angle of the polarization vector. We obtain that the corona+disk configuration studied in this work can account for the high degree of polarization of gamma-rays detected in galactic black holes without the need of a relativistic jet; specific predictions are made for sources in a low-hard state. In particular, the model is applied to the transient source \xtee; we show that if a new outburst of \xte is observed, then its gamma-ray polarization should be measurable by future instruments, such as ASTRO-H or the proposed ASTROGAM.