恒星物理研究组知识库

Observations show that the accretion flows in low-luminosity active galactic nuclei probably have a two-component structure with an inner, hot, optically thin, advection-dominated accretion flow (ADAF) and an outer, truncated, cool, optically thick accretion disk. As shown by Taam et al., the truncation radius as a function of mass accretion rate is strongly affected by including the magnetic field within the framework of disk evaporation model, i.e., an increase in the magnetic field results in a smaller truncation radius of the accretion disk. In this work, we calculate the emergent spectrum of an inner ADAF + an outer truncated accretion disk around a supermassive black hole based on the prediction by Taam et al.. It is found that an increase in the magnetic field from beta = 0.8 to beta = 0.5 (with magnetic pressure p(m) = B-2/8 pi = (1 - beta)p(tot), p(tot) = p(gas) + p(m)) results in a factor of similar to 8.7 increase in the luminosity from the truncated accretion disk. Meanwhile, results of the peak emission of the truncated accretion disk shift toward a a factor of similar to 5 higher frequency. We found that the equipartition of gas pressure to magnetic pressure, i.e., beta = 0.5, failed to explain the observed anti-correlation between (L2-10) (keV)/L-Edd and the bolometric correction kappa(2-10 keV) (with kappa(2-10 keV) = L-bol/L2-10 keV). The emergent spectra for larger values of beta = 0.8 or beta = 0.95 can explain the observed L2-10 keV/L-Edd-kappa(2-10 keV) correlation. We argue that in the disk evaporation model, the electrons in the corona are assumed to be heated only by a transfer of energy from the ions to electrons via Coulomb collisions, which is reasonable for accretion with a lower mass accretion rate. Coulomb heating is the dominated heating mechanism for the electrons only if the magnetic field is strongly sub-equipartition, which is roughly consistent with observations.