A study of the strong gravity region of the black hole in GS 1354-645

TL;DR

This study tests the Kerr black hole hypothesis by fitting X-ray spectra of GS 1354-645 with a modified gravity model, constraining deviations from Einstein's predictions and confirming a high spin parameter.

Contribution

It provides observational constraints on deviations from Kerr black holes using X-ray spectral fitting with the Johannsen metric.

Findings

Black hole spin measured as a* > 0.975

Deformation parameters constrained within specific ranges

Results consistent with Kerr black hole hypothesis

Abstract

It is thought that the spacetime metric around astrophysical black holes is well described by the Kerr solution of Einstein's gravity. However, a robust observational evidence of the Kerr nature of these objects is still lacking. Here we fit the X-ray spectrum of the stellar-mass black hole in GS 1354-645 with a disk reflection model beyond Einstein's gravity in order test the Kerr black hole hypothesis. We consider the Johannsen metric with the deformation parameters $\alpha_{13}$ and $\alpha_{22}$. The Kerr metric is recovered for $\alpha_{13} = \alpha_{22} = 0$. For $\alpha_{22} = 0$, our measurements of the black hole spin and of the deformation parameter $\alpha_{13}$ are $a_* > 0.975$ and $-0.34 < \alpha_{13} < 0.16$, respectively. For $\alpha_{13} = 0$, we find $a_* > 0.975$ and $-0.09 < \alpha_{22} < 0.42$. All the reported uncertainties are at 99% of confidence level for two relevant parameters.