http://arxiv.org/abs/1607.00928
The radio luminosity, $L_R$, of the low-luminosity active galactic nucleus (AGN) NGC~7213 weakly correlates with its 2–10 keV X-ray luminosity, $L_X$, when $L_X$ is low, and apparently strongly when $L_X$ is high. Such a hybrid correlation in an individual AGN is unexpected as it deviates from the fundamental plane of AGN activity. On the other hand, a similar correlation pattern is present in the black-hole X-ray binary H1743–322, where it has been modelled as due to switching between different modes of accretion. For NGC~7213, we propose that its $L_R$–$L_X$ correlation is due to the presence of a luminous hot accretion flow, which radiative efficiency is sensitive to the accretion rate. Given the low luminosity of the source, $L_X\sim 10^{-4}$ of the Eddington luminosity, the viscosity parameter is determined to be small in NGC 7213, $\alpha\approx 0.01$. We also examine the broad-band spectrum from radio to X-rays, the time lag between the radio and X-ray light curves, and the implied size and the Lorentz factor of the radio jet. In our model, the source is expected to enter into a two-phase accretion regime when $L_X$ is $> 1.5 \times 10^{42}\, {\rm erg\,s^{-1}}$. When it happens, we predict a softening of the X-ray spectrum with the increasing flux and a steep radio/X-ray correlation.
F. Xie, A. Zdziarski, R. Ma, et. al.
Tue, 5 Jul 16
77/80
Comments: 8 pages, 5 figures. Sumbitted to MNRAS. Comments are welcome
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