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

A. Merloni & A. C. Fabian
IoA, Cambridge


The spectral energy distributions of galactic black holes in the low/hard state and of low-luminosity AGN 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. If angular momentum transport in the disc is due to magnetic  turbulent stresses and both magnetic energy density and effective viscous stresses inside the disc
are proportional to the gas pressure, we show that the relative fraction of the power liberated in  the corona must increase as the accretion rate decreases. Furthermore, we discuss reasons 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.