Light bending scenario for accreting black holes in X-ray polarimetry

TL;DR

This paper models the polarization of X-ray reflection from accreting black holes using a lamp-post scheme, exploring how parameters like spin and viewing angle affect polarization, with implications for future X-ray polarimetry missions.

Contribution

It introduces a detailed Monte Carlo simulation of X-ray polarization in accreting black holes considering general relativity effects and evaluates observability with upcoming missions.

Findings

Polarization degree increases above 20 keV, especially in the Compton hump region.

Higher black hole spin correlates with increased polarization.

Lower source height and observer inclination lead to higher polarization.

Abstract

We discuss a model of an X-ray illuminating source above an accretion disk of a rotating black hole. Within the so called lamp-post scheme we compute the expected (observed) polarization properties of the radiation reaching an observer. We explore the dependencies on model parameters, employing Monte Carlo radiation transfer computations of the X-ray reflection on the accretion disk and taking general relativity effects into account. In particular, we discuss the role of the black hole spin, of the observer viewing angle, and of the primary X-ray source distance from the black hole. We give several examples of the resulting polarization degree for two types of exemplary objects - active galactic nuclei and Galactic black holes. In order to assess potential observability of the polarization features, we assume the sensitivity of the proposed New Hard X-ray Mission (NHXM). We examine the energy range from several keV to ~50 keV, so the iron-line complex and the Compton hump are included in our model spectra. We find the resultant polarization degree to increase at the higher end of the studied energy band, i.e. at >~20 keV. Thus, the best results for polarimetry of reflection spectra should be achieved at the Compton hump energy region. We also obtain higher polarization degree for large spin values of the black hole, small heights of the primary source, and low inclination angles of the observer.