The advection-dominated accretion flow+thin accretion disk model for two low-luminosity active galactic nuclei: M81 and NGC 4579

It was found that advection-dominated accretion flow （ADAF）＋thin disk model calculations can reproduce the observed spectral energy distributions （SEDs） of two low-luminosity active galactic nuclei （AGNs）, provided they are accreting at -0.01-0.03 Eddington rates and the thin disks are truncated to ADAFs at -100Rs （Rs is the Schwarzschild radius） for M81 and NGC 4579 （Quataert et al. 1999）. However, the black hole masses adopted in their work are about one order of magnitude lower than recent measurements on these two sources. Adopting the well estimated black hole masses, our ADAF＋thin disk model calculations can reproduce the observed SEDs of these two low- luminosity AGNs, if the black hole is accreting at 2.5×10^-4 Eddington rates with the thin disk truncated at Rtr=120Rs for M81（m= 3.3×10^-3 and Rtr=80Rs are required for NGC 4579）. The transition zones with temperature from the thin disk with ,-10^4- 10^5 to ,-10^9-10^10K in the ADAF will inevitably emit thermal X-ray lines, which provides a useful diagnosis of their physical properties. The observed widths of the thermal X-ray iron lines at -6.8 keV are consistent with Doppler broadening by Keplerian motion of the gases in the transition zones at ,-100Rs. We use the structure of the transition zone between the ADAF and the thin disk derived by assuming the turbulent diffusive heat mechanism to calculate their thermal X-ray line emission with the standard software package Astrophysical Plasma Emission Code （APEC）. Comparing them with the equivalent widths of the observed thermal X-ray iron lines in these two sources, we find that the turbulent diffusive heat mechanism seems to be unable to reproduce the observed thermal X-ray line emission. The test of the evaporation model for the accretion mode transition with the observed thermal X-ray line emission is briefly discussed.

Ignition of neutrino-dominated accretion disks

The vertical structure of neutrino-dominated accretion flows （NDAFs） in spherical coordinates is revisited. We define the ratio of the neutrino cooling rate to the viscous heating rate per unit volume, f=-qJqvis. The ignition region is presented when f〉0.5. The solutions show that NDAF is significantly thick. The ignition region is determined by the mass accretion rate and the vis- cosity parameter, which can be considered as the neutrino radiation-dominated region.

Origin of power-law X-ray emission in the steep power-law state of X-ray binaries

We present a new explanation for the origin of the steep power-law（SPL） state of X-ray binaries.The power-law component of X-ray emission is the synchrotron radiation of relativistic electrons in highly magnetized compact spots orbiting near the inner stable circular orbit of a black hole.It has a hard spectrum that extends to above MeV energies,which is determined by the electron acceleration rate.These photons are then down-scattered by the surrounding plasma to form an observed steep spectrum.We discuss the relevance of the model to high-frequency quasi-periodic oscillations and the extremely high luminosity of the SPL state.