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

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.