2015MNRAS.447.1692Y

We investigate the observed correlation between the 2-10 keV X-ray luminosity (in unit of the Eddington luminosity; lX ≡ LX/LEdd) and the photon index (Γ) of the X-ray spectrum for both black hole X-ray binaries (BHBs) and active galactic nuclei (AGNs). We construct a large sample, with 10^- 9^ ≲ lX ≲ 10^- 1^. We find that Γ is positively and negatively correlated with lX when lX ≳ 10^- 3^ and 10^- 6.5^ ≲ lX ≲ 10^- 3^, respectively, while Γ is nearly a constant when lX ≲ 10^- 6.5^. We explain the above correlation in the framework of a coupled hot accretion flow-jet model. The radio emission always comes from the jet while the X-ray emission comes from the accretion flow and jet when lX is above and below 10^–6.5, respectively. More specifically, we assume that with the increase of mass accretion rate, the hot accretion flow develops into a clumpy and further a disc-corona two-phase structure because of thermal instability. We argue that such kind of two-phase accretion flow can explain the observed positive correlation.