Constraining the Johannsen deformation parameter ɛ3 with black hole x-ray data

We extend our previous work on tests of the Kerr black hole hypothesis using x-ray reflection spectroscopy and we infer observational constraints on the Johannsen deformation parameter ɛ₃ . We analyze a NuSTAR observation of the stellar-mass black hole in GS 1354-645, Suzaku data of the supermassive black holes in Ark 564, 1H0419-577, and PKS 0558-504, and some simultaneous XMM-Newton and NuSTAR observations of the supermassive black hole in MCG-6-30-15. All our measurements of ɛ₃ are consistent with the Kerr metric and we find quite strong constraints from Ark 564 and MCG-6-30-15. We discuss the implications of our results and the next steps to improve our tests.