The X-ray spectral evolution and radio–X-ray correlation in radiatively efficient black-hole sources

Abstract

We explore X-ray spectral evolution and radio–X-ray correlation simultaneously for four X-ray binaries (XRBs). We find that hard X-ray photon indices, Γ, are anti- and positively correlated to X-ray fluxes when the X-ray flux, F_3–9keV, is below and above a critical flux, F_X,crit, which may be regulated by ADAF and disk-corona respectively. We find that the data points with anti-correlation of Γ-F_3–9keV follow the universal radio–X-ray correlation of F_R ∝ F_X^b (b ~ 0.5-0.7), while the data points with positive X-ray spectral evolution follow a steeper radio–X-ray correlation (b ~ 1.4, the so-called ‘outliers track’). The bright active galactic nuclei (AGNs) share similar X-ray spectral evolution and radio–X-ray correlation as XRBs in ‘outliers’ track, and we present a new fundamental plane of log L_R=1.59^+0.28_−0.22 log L_X−0.22^+0.19_−0.20 log M_BH−28.97^+0.45_−0.45 for these radiatively efficient BH sources.