A luminous hot accretion flow in the low-luminosity active galactic nucleus NGC 7213

Abstract

The active galactic nucleus (AGN) NGC 7213 shows a complex correlation between the monochromatic radio luminosity $L_R$ and the 2--10 keV X-ray luminosity $L_X$, i.e. the correlation is unusually weak with $p\sim 0$ (in the form $L_R\propto L_X^p$) when $L_X$ is below a critical luminosity, and steep with $p>1$ when $L_X$ is above that luminosity. Such a hybrid correlation in individual AGNs is unexpected as it deviates from the Fundamental Plane of AGN activity. Interestingly, a similar correlation pattern is observed in the black hole X-ray binary H1743--322, where it has been modelled by switching between different modes of accretion. We propose that the flat $L_R$--$L_X$ correlation of NGC 7213 is due to the presence of a luminous hot accretion flow, an accretion model whose 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, $\alpha\approx 0.01$. We also modelled the broad-band spectrum from radio to $\gamma$-rays, the time lag between the radio and X-ray light curves, and the implied size and the Lorentz factor of the radio jet. We predict that NGC 7213 will enter into a two-phase accretion regime when $L_X > 1.5 \times 10^{42}\, {\rm erg\,s^{-1}}$. When this happens, we predict a softening of the X-ray spectrum with the increasing flux and a steep radio/X-ray correlation.