Astrophys. J., 777, 102 (2013/November-2)
The disk evaporation model for the spectral features of low-luminosity active galactic nuclei.
QIAO E., LIU B.F., PANESSA F. and LIU J.Y.
Abstract (from CDS):
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 β = 0.8 to β = 0.5 (with magnetic pressure pm= B 2/8π = (1 - β)ptot, ptot= pgas+ pm) results in a factor of ∼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 ∼5 higher frequency. We found that the equipartition of gas pressure to magnetic pressure, i.e., β = 0.5, failed to explain the observed anti-correlation between L_2-10 keV/LEdd_ and the bolometric correction κ_2-10 keV_(with κ_2-10 keV_= Lbol/L_2-10 keV_). The emergent spectra for larger values of β = 0.8 or β = 0.95 can explain the observed L_2-10 keV/LEdd–κ2-10 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.
accretion, accretion disks - black hole physics - galaxies: active - X-rays: galaxies
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