Difficulties of quantitative tests of the Kerr-hypothesis with X-ray observations of mass accreting black holes

Abstract

X-ray studies of stellar mass black holes in X-ray binaries and mass-accreting supermassive black holes in Active Galactic Nuclei have achieved a high degree of maturity and have delivered detailed information about the astrophysical sources and the physics of black hole accretion. In this article, I review recent progress made towards using the X-ray observations for testing the "Kerr hypothesis" that the background spacetimes of all astrophysical quasi-stationary black holes are described by the Kerr metric. Although the observations have indeed revealed clear evidence for relativistic effects in strong-field gravity, quantitative tests of the Kerr hypothesis still struggle with theoretical and practical difficulties. In this article, I describe several recently introduced test metrics and review the status of constraining the background spacetimes of mass accreting stellar mass and supermassive black holes with these test metrics. The main conclusion of the discussion is that astrophysical uncertainties are large compared to the rather small observational differences between the Kerr and non-Kerr metrics precluding quantitative constraints on deviations from the Kerr metric at this point in time. I conclude with discussing future progress enabled by more detailed numerical simulations and by future X-ray spectroscopy, timing, polarimetry, and interferometry missions.

Search for ultralight bosons in Cygnus X-1 with Advanced LIGO

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Does the gravitomagnetic monopole exist? A clue from a black hole x-ray binary

“…However, one or more of these methods may not be entirely reliable due to additional physical complexities. Some of these complexities may be due to the following reasons (e.g., [43] discusses how difficult it is to test the Kerr metric with X-ray observations).…”

Towards Precision Measurements of Accreting Black Holes Using X-Ray Reflection Spectroscopy

“…In the calculations of the relativistic reflection spectrum of an accretion disk, we normally assume Einstein's theory of GR, and thus: i) the background metric is described by the Kerr solution, ii) massive and massless particles follow the geodesics of the spacetime (Weak Equivalence Principle), and iii) atomic physics in the accretion disk is the same as that we can study in our laboratories on Earth (Local Lorentz Invariance and Local Position Invariance) 13 . Relaxing one of these assumptions, we can use X-ray reflection spectroscopy to test Einstein's theory of GR in the strong field regime (Johannsen 2016, Bambi 2017b, Krawczynski 2018.…”