2018ApJ...865..134X

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 α₁₃ and α₂₂. The Kerr metric is recovered for α₁₃=α₂₂=0. For α₂₂ = 0, our measurements of the black hole spin and of the deformation parameter α₁₃ are a_*> 0.975 and -0.34 < α₁₃ < 0.16, respectively. For α₁₃ = 0, we find a_* > 0.975 and -0.09 < α₂₂ < 0.42. All the reported uncertainties are at 99% of confidence level for two relevant parameters.