Probing the near-horizon region of Cygnus X-1 with Suzaku and NuSTAR

TL;DR

This study analyzes Suzaku and NuSTAR data of Cygnus X-1 to test the strong-field predictions of general relativity near a black hole, constraining deviations from the Kerr metric.

Contribution

First to constrain the Johannsen deformation parameter using combined Suzaku and NuSTAR spectra of Cygnus X-1.

Findings

Constraints on the Kerr metric are not very stringent.

Suzaku's high energy resolution aids in spectral analysis.

Cygnus X-1 remains a complex source for testing gravity.

Abstract

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 Suzaku and NuSTAR spectra of the black hole binary Cygnus X-1 using RELXILL_NK. Unlike our previous study on Cygnus X-1 with NuSTAR data, here we are able to constrain the Johannsen deformation parameter $\alpha_{13}$. However, despite the high energy resolution near the iron line provided by Suzaku, 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.