Testing General Relativity's No-Hair Theorem with X-Ray Observations of Black Holes

TL;DR

This study uses detailed ray tracing simulations of X-ray observations to test the no-hair theorem of General Relativity by comparing Kerr and non-Kerr black hole metrics, focusing on rapidly spinning black holes.

Contribution

It introduces a comprehensive simulation approach combining spectroscopy, timing, and polarization data to distinguish Kerr black holes from alternative models.

Findings

X-ray observations can exclude large regions of non-Kerr parameter space.

Rapidly spinning black holes provide the best tests for deviations from Kerr.

Some non-Kerr metrics remain nearly indistinguishable from Kerr in certain conditions.

Abstract

Despite its success in the weak gravity regime, General Relativity (GR) has yet to be verified in the regime of strong gravity. In this paper, we present the results of detailed ray tracing simulations aiming at clarifying if the combined information from X-ray spectroscopy, timing, and polarization observations of stellar mass and supermassive black holes can be used to test GR's no-hair theorem. The latter states that stationary astrophysical black holes are described by the Kerr-family of metrics with the black hole mass and spin being the only free parameters. We use four "non-Kerr metrics", some phenomenological in nature and others motivated by alternative theories of gravity, and study the observational signatures of deviations from the Kerr metric. Particular attention is given to the case when all the metrics are set to give the same Innermost Stable Circular Orbit (ISCO) in quasi-Boyer Lindquist coordinates. We give a detailed discussion of similarities and differences of the observational signatures predicted for BHs in the Kerr metric and the non-Kerr metrics. We emphasize that even though some regions of the parameter space are nearly degenerate even when combining the information from all observational channels, X-ray observations of very rapidly spinning black holes can be used to exclude large regions of the parameter space of the alternative metrics. Although it proves difficult to distinguish between the Kerr and non-Kerr metrics for some portions of the parameter space, the observations of very rapidly spinning black holes like Cyg X-1 can be used to rule out large regions for several black hole metrics.