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 accretion flow available at the outer boundary can finally fall on to the black hole, while most of it is lost in the outflow; (ii) electrons can directly receive a large fraction of the viscously dissipated energy in the accretion flow (i.e. δ ∼ 0.1–0.5). The radiative efficiency of the hot accretion flow when these two findings are taken into account has not yet been systematically studied, and this is the subject of our paper. We consider two regimes of the hot accretion model: advection‐dominated accretion flows that lie in the regime of the low accretion rate, ≲10α2L Edd /c2, and the luminous hot accretion flows (LHAFs) that lie above this accretion rate. For the LHAFs, we assume that the accretion flow has a two‐phase structure above a certain accretion rate, and we adopt a simplification in our calculation of the dynamics. Our results indicate that the radiative efficiency of hot accretion flow increases with the accretion rate and that it is greatly enhanced by the direct viscous heating to electrons, compared to the previous case of δ ≪ 1. When the accretion rate is high, the radiative efficiency of the hot accretion flow is comparable to that of a standard thin disc. We present fitting formulae of radiative efficiency as a function of accretion rate for various values of δ.

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 come 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.

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We present the X-ray timing results of the new black hole candidate (BHC) MAXI J1535-571 during its 2017 outburst from Hard X-ray Modulation Telescope (Insight -HXMT) observations taken from 2017 September 6 to 23. Following the definitions given by Belloni (2010), we find that the source exhibits state transitions from Low/Hard state (LHS) to Hard Intermediate state (HIMS) and eventually to Soft Intermediate state (SIMS). Quasi-periodic oscillations (QPOs) are found in the intermediate states, which suggest different types of QPOs. With the large effective area of Insight -HXMT at high energies, we are able to present the energy dependence of the QPO amplitude and centroid frequency up to 100 keV which is rarely explored by previous satellites. We also find that the phase lag at the type-C QPOs centroid frequency is negative (soft lags) and strongly correlated with the centroid frequency. By assuming a geometrical origin of type-C QPOs, the source is consistent with being a high inclination system.

Both numerical simulations and observations indicate that in an advection-dominated accretion flow most of the accretion material supplied at the outer boundary will not reach the inner boundary. Rather, they are lost via outflow. Previously, the influence of outflow on the dynamics of inflow is taken into account only by adopting a radius-dependent mass accretion rate $\dot{M}=\dot{M}_0 (r/r_{\rm out})^s$ with $s>0$. In this paper, based on a 1.5 dimensional description to the accretion flow, we investigate this problem in more detail by considering the interchange of mass, radial and azimuthal momentum, and the energy between the outflow and inflow. The physical quantities of the outflow is parameterized based on our current understandings to the properties of outflow mainly from numerical simulations of accretion flows. Our results indicate that under reasonable assumptions to the properties of outflow, the main influence of outflow has been properly included by adopting $\dot{M}=\dot{M}_0 (r/r_{\rm out})^s$.Comment: 16 pages, 5 figures. accepted for publication in Ap