Probing the near-horizon region of Cygnus X-1 with S u z a k u and N u S T A R

Astrophysical black holes are ideal laboratories for testing Einstein's theory of general relativity in the strong field regime. In this manuscript, we present an analysis of S u z a k u and N u S T A R spectra of the black hole binary Cygnus X-1 using relxill_nk. Unlike our previous study on Cygnus X-1 with N u S T A R data, here we are able to constrain the Johannsen deformation parameter α₁₃. However, despite the high energy resolution near the iron line provided by S u z a k u , our constraints on the Kerr metric from Cygnus X-1 are not very stringent in comparison with those that have been obtained from other sources, confirming that Cygnus X-1 is quite a complicated source.