Tests of General Relativity in the Strong Gravity Regime Based on X-Ray Spectropolarimetric Observations of Black Holes in X-Ray Binaries

TL;DR

This study investigates whether X-ray spectropolarimetric observations of black holes can differentiate between General Relativity and alternative metrics, highlighting the challenges due to similar observational signatures in many cases.

Contribution

The paper develops a numerical code to compare GR and non-GR black hole spacetimes using X-ray spectropolarimetry, aiding in strong gravity tests.

Findings

Energy spectra and polarization depend on spacetime metric.

GR and non-GR signatures are often observationally similar.

Distinguishing metrics is challenging in many parameter regions.

Abstract

Although General Relativity (GR) has been tested extensively in the weak gravity regime, similar tests in the strong gravity regime are still missing. In this paper we explore the possibility to use X-ray spectropolarimetric observations of black holes in X-ray binaries to distinguish between the Kerr metric and the phenomenological metrics introduced by Johannsen and Psaltis (2011) (which are not vacuum solutions of Einstein's equation) and thus to test the no-hair theorem of GR. To this end, we have developed a numerical code that calculates the radial brightness profiles of accretion disks and parallel transports the wave vector and polarization vector of photons through the Kerr and non-GR spacetimes. We used the code to predict the observational appearance of GR and non-GR accreting black hole systems. We find that the predicted energy spectra and energy dependent polarization degree and polarization direction do depend strongly on the underlying spacetime. However, for large regions of the parameter space, the GR and non-GR metrics lead to very similar observational signatures, making it difficult to observationally distinguish between the two types of models.