2016MNRAS.463.2287X

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 ∼ 0 (in the form L_ R_∝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 ∼ 10^–4 of the Eddington luminosity, the viscosity parameter is determined to be small, α ≃ 0.01. We also modelled the broad-band spectrum from radio to γ-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 x 10⁴² 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.