Testing general relativity with NuSTAR data of galactic black holes. II
Abstract
Abstract General relativity predicts the spacetime metric around an astrophysical black hole to be described by the Kerr solution, which relates to a massive rotating black hole without any residual charge. In a previous paper, we analyzed the NuSTAR observations of six X-ray binaries to obtain constraints on the deformation parameter $\alpha _{13}$ using a state-of-the-art relativistic model. In this work, we continue analyzing NuSTAR observations of four more X-ray binaries; two of which, namely Swift J174540.7-290015 and Swift J174540.2-290037, are X-ray transients very close to the supermassive black hole at the center of our Galaxy. The other two sources have complicated absorption, which is accounted for by time- and flux-resolved spectroscopy. The observation of MAXI J1631-479 is divided into two parts to account for the sudden increase in flux. The V404 Cygni spectra, obtained by combining two consecutive observations, are divided into five flux states and also account for absorption by quantifying the excess flux in the energy range of 6.5–$7.0$ keV. The constraints obtained are consistent with the Kerr hypothesis and are comparable with those obtained in previous studies and those from gravitational events. This work shows that even highly absorbed sources can be used for testing the Kerr hypothesis, which is possible with the careful data reduction and subsequent data analysis.