Coronal outflow dominated accretion discs: a new possibility for low-luminosity black holes?

Abstract

The spectral-energy distributions of galactic black holes in the low/hard state and of low-luminosity active galactic nuclei (AGNs) possess many common features, the most prominent being: compact, flat- (or inverted-)spectrum radio cores with high brightness temperatures; excess red and infrared emission, often correlated with the radio flux; an extremely weak (or absent) quasi-thermal hump and a hard-X-ray power-law with high-energy cut-off. These sources are thought to be accreting at low rates, and advection- (or convection-)dominated accretion flows are usually considered the best candidates to explain them. Here we present an alternative possibility, involving strong, unbound, magnetic coronae generated by geometrically thin, optically thick accretion discs at low accretion rates. First we show that, if angular momentum transport in the disc is due to magnetic turbulent stresses, the magnetic energy density and effective viscous stresses inside the disc are proportional to the geometric mean of the total (gas plus radiation) and gas pressure. Therefore the corona is less powerful in a radiation-pressure dominated disc, and the relative fraction of the power liberated in the corona increases as the accretion rate decreases. Furthermore, we discuss why energetically dominant coronae are ideal sites for launching powerful jets/outflows, both MHD- and thermally driven. In analysing the spectral properties of such coronal-outflow dominated accretion discs, we reach the important conclusion that if the jet/outflow is, as is likely, radiatively inefficient, then so is the source overall, even without advection of energy into the black hole being relevant for the dynamics of the accretion flow.