The disk evaporation model for the spectral features of low-luminosity active galactic nuclei

Abstract

Observations show that the accretion flows in low-luminosity active galactic nuclei (LLAGNs) probably have a two-component structure with an inner ADAF and an outer truncated accretion disk. As shown by Taam et al. (2012), 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 of 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. (2012). It is found that an increase of the magnetic field from ${\hat{I}}^2=0.8$ to ${\hat{I}}^2=0.5$ (with magnetic pressure $p_{\mathrm{m}}=B^2/8\ddot{I}€=(1-{\hat{I}}^2)p_{\mathrm{tot}}$, $p_{\mathrm{tot}}=p_{\mathrm{gas}}+p_{\mathrm{m}}$) results in an increase of $\sim 8.7$ times of the luminosity from the truncated accretion disk. We found that the equipartition of gas pressure to magnetic pressure, i.e., ${\hat{I}}^2=0.5$, failed to explain the observed anti-correlation between $L_{2-10keV}/L_{\mathrm{Edd}}$ and the bolometric correction $\hat{I}{º}_{2-10keV}$ (with $\hat{I}{º}_{2-10keV}=L_{\mathrm{bol}}/L_{2-10keV}$). The emergent spectra for larger value ${\hat{I}}^2=0.8$ or ${\hat{I}}^2=0.95$ can well explain the observed $L_{2-10keV}/L_{\mathrm{Edd}}$-$\hat{I}{º}_{2-10keV}$ 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 the 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.

Accepted for publication by ApJ