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

Abstract

A B S T R A C TThe spectral-energy distributions of galactic black holes in the low/hard state and of lowluminosity 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.

Reconnection in Marginally Collisionless Accretion Disk Coronae

“…In the sandwich model, the torque is ultimately exerted on the disk since the footpoints of the coronal field and most of the rotational inertia are rooted there. It seems likely that f is closer to unity than to zero when the nonthermal power law dominates the spectrum (Merloni & Fabian 2002). Then fṀ GMR ð Þ 1/2 % 4 R 2 HhÀB R B /4 i corona .…”

On the Origin of Black Hole X‐Ray Emission in Quiescence:ChandraObservations of XTE J1550−564 and H1743−322

“…In the magnetic corona model, active regions above the disk are responsible for producing the hard X-ray emission and constitute the base of the outflow (Merloni & Fabian 2002). In that model, the X-ray spectrum is expected to soften at a lower accretion rate as the accretion power is carried away into the jets rather that being used to heat the electrons (Merloni & Fabian 2002).…”

Radiation Pressure Instability Driven Variability in the Accreting Black Holes

“…Recently, Merloni & Fabian (2002) proposed that the coronal fraction may drop for high accretion rates, being the most efficient in the low state of the source. When either an outflow or a corona is strong enough ( f cor > 0:55 and/or A > 0:25), the outbursts do not occur and the disk produces only flickering.…”