Global Accretion Properties of Black Hole X-Ray Binaries: A Phenomenological Perspective

Abstract

Black hole X-ray binaries (BHXBs) show rich phenomenology in the spectral and timing properties. We collected the spectral data of 20 BHXBs from the literature across different spectral states. The spectral properties are studied in the forms of the inner disc temperature ($T_{\rm in}$), photon index ($Γ$), hot electron temperature ($kT_{\rm e}$), X-ray flux ($F_{\rm X}$) and luminosity ($L_{\rm X}$). We studied various correlations among different spectral parameters to understand the accretion process on a global scale. In the thermal soft states (TSS), we find most of the sources followed $F_{\rm disc} \propto T_{\rm in}^4$ relation. A `V'-shaped correlation is found between $Γ$ and total luminosity ($L_{\rm tot}$) in the hard Comptonized state (HCS). The Comptonized luminosity is observed to be correlated with the disc luminosity in the HCS and TSS. No notable correlation is observed in the intermediate state (IMS). The evolution of the inner disc radius ($R_{\rm in}$) is unclear in the HCS and IMS. We also discuss how the hot electron temperature changes with other spectral parameters. We observe that the iron line flux correlates with disc and Comptonized fluxes. The strength of the reprocessed emission is found to vary across spectral states.