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, 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 alpha(*) > 0.975 and -0.34 < alpha(13 )< 0.16, respectively. For alpha(13 )= 0, we find alpha* > 0.975 and -0.09 < alpha(22) < 0.42. All the reported uncertainties are at 99% of confidence level for two relevant parameters.