Over the past few years, our understanding of hot accretion flows has been improved significantly by two findings: (i) only a small fraction of the ac

We investigate the observed correlation between the 2–10 keV X-ray luminosity (in unit of the Eddington luminosity; l_X ≡ L_X/L_Edd) 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} ≲ l_X ≲ 10^{− 1}. We find that Γ is positively and negatively correlated with l_X when l_X ≳ 10^{− 3} and 10^{− 6.5} ≲ l_X ≲ 10^{− 3}, respectively, while Γ is nearly a constant when l_X ≲ 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 l_X 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.

We analyze the X-ray, optical, and mid-infrared data of a “changing-look” Seyfert galaxy SDSS J155258+273728 at z ≃ 0.086. Over a period of one decade (2009–2018), its broad Hα line intensity increased by a factor of ∼4. Meanwhile, the X-ray emission in 2014 as observed by Chandra was about five times brighter than that in 2010 by Suzaku, and the corresponding emissions in the V-band, mid-infrared W1 band brighten by ∼0.18, 0.32 mag, respectively. Moreover, the absorption in X-rays is moderate and stable, i.e., N H ∼ 10 21 cm − 2 , but the X-ray spectrum becomes harder in the 2014 Chandra bright state (i.e., photon index Γ = 1.52 − 0.06 + 0.06 ) than that of the 2010 Suzaku low state ( Γ = 2.03 − 0.21 + 0.22 ). With an Eddington ratio being lower than a few percent, the inner region of the accretion disk in SDSS J155258+273728 is likely a hot accretion flow. We then compile from literature the X-ray data of “changing-look” active galactic nuclei (AGNs), and find that they generally follow the well-established “V”-shaped correlation in AGNs, that is, above a critical turnover luminosity the X-ray spectra soften with the increasing luminosity, and below that luminosity the trend is reversed in the way of “harder when brighter.” This presents direct evidence that CL-AGNs have distinctive changes in not only the optical spectral type, but also the X-ray spectral shape. The similarity in the X-ray spectral evolution between CL-AGNs and black hole X-ray binaries indicates that the observed CL-AGNs phenomena may relate to the state transition in accretion physics.

Two types of correlations between the radio and X-ray luminosities (L_R and L_X) have been found in black hole X-ray binaries. For some sources, they follow the ‘original’ type of correlation which is described by a single power law. Later it was found that some other sources follow a different correlation consisting of three power-law branches, with each branch having different power-law indexes. In this work, we explain these two types of correlation under the coupled accretion–jet model. We attribute the difference between these two types of sources to the different value of viscosity parameter α. One possible reason for different α is the different configuration of magnetic field in the accretion material coming from the companion stars. For the ‘single power-law’ sources, their α is high; so their accretion is always in the mode of advection-dominated accretion flow (ADAF) for the whole range of X-ray luminosity. For those ‘hybrid power-law’ sources, the value of α is small so their accretion mode changes from an ADAF to a luminous hot accretion flow, and eventually to two-phase accretion as the accretion rate increases. Because the dependence of radiative efficiency on the mass accretion rate is different for these three accretion modes, different power-law indexes in the L_R–L_X correlation are expected. Constraints on the ratio of the mass-loss rate into the jet and the mass accretion rate in the accretion flow are obtained, which can be tested in future by radiative magnetohydrodynamic numerical simulations of jet formation.

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