Magnetic reconnection: flares and coronal heating in active galactic nuclei

Abstract

ABSTRA C T A magnetically structured accretion disc corona, generated by buoyancy instability in the disc, can account for observations of flare-like events in active galactic nuclei. We examine how Petschek magnetic reconnection, associated with MHD turbulence, can result in a violent release of energy and heat the magnetically closed regions of the corona up to canonical X-ray emitting temperatures. X-ray magnetic flares, the after effect of the energy released in slow shocks, can account for the bulk of the X-ray luminosity from Seyfert galaxies and consistently explain the observed short-time-scale variability. process directly into the corona outside the disc. Magnetic recon- nection can be responsible for the rapid dissipation of magnetic energy though a field-aligned electric potential in thin current sheets. We, here, describe how the observationally required large heating rates per unit mass, in the optically thin gas of a hot coronal region, can be accounted for by an ion-acoustic instability in the context of slow shocks associated with Petschek-type reconnection and give rise to flare-like events. We estimate that such a process may be responsible for the heating of the coronal plasma in the active regions of an AGN to a level where it emits X-rays at a temperature of ,10 9 K even in the presence of inverse Compton and synchro- tron cooling processes. Magnetic buoyancy, which drives magnetic flux out of the disc, naturally decreases the plasma density to the point where the active region can reconnect very efficiently. X-ray observations imply that the coronal plasma is very tenuous with a density much lower than that of the underlying disc. We show that the energetics of such flares are consistent with the X-ray lumin- osities of typical Seyfert galaxies. The onset of magnetic flares through slow shocks associated with Petschek reconnection, should occur over time-scales comparable to those required to explain