Testing the Kerr nature of the supermassive black hole in Ark 564

TL;DR

This study tests Einstein's general relativity in the strong gravity regime by analyzing X-ray spectra of the supermassive black hole in Ark 564, constraining deviations from the Kerr metric with high confidence.

Contribution

It introduces a method to test the Kerr nature of black holes using X-ray spectral fitting with a beyond-Einstein gravity model, providing new constraints on deviations.

Findings

Black hole in Ark 564 is consistent with Kerr metric.

Constraints on deformation parameters are tight at 99% confidence.

Results support Einstein's theory in strong gravity regime.

Abstract

Einstein's theory of general relativity has been extensively tested in weak gravitational fields, mainly with experiments in the Solar System and observations of radio pulsars, and current data agree well with the theoretical predictions. Nevertheless, there are a number of scenarios beyond Einstein's gravity that have the same predictions for weak fields and present deviations only when gravity becomes strong. Here we try to test general relativity in the strong field regime. We fit the X-ray spectrum of the supermassive black hole in Ark 564 with a disk reflection model beyond Einstein's gravity, and we are able to constrain the black hole spin $a_*$ and the Johannsen deformation parameters $\alpha_{13}$ and $\alpha_{22}$ separately. For $\alpha_{22} = 0$, we find $a_* > 0.96$ and $-1.0 < \alpha_{13} < 0.2$ with a 99% confidence level. For $\alpha_{13} = 0$, we get $a_* > 0.96$ and $-0.1 < \alpha_{22} < 0.9$ with a 99% confidence level. Our measurements are thus consistent with the hypothesis that the supermassive compact object in Ark 564 can be described by the Kerr metric.