Abstract 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10-4Eddington 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 ∼ 104 − 105 to ∼ 109 − 1010 K 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.

Keywords accretion, accretion disks—black hole physics—galaxies:active—radiation mechanisms: thermal— galaxies: individual (M81, NGC 4579)

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