Magnetic Pumping in Accretion Disk Coronae

Abstract

Most microquasar models involve a hot plasma in a corona above the disk or at the base of the jet. The Accretion-Ejection Instability (AEI) occurring in magnetized disks leads to the growth of a spiral density wave which can explain low frequency QPOs. It has already been shown to be very efficient in extracting accretion energy from the disk and emitting it upward in the corona as Alfven waves. Here we present a simple mechanism which also allows the AEI to excite coronal ions. This heating is due to magnetic pumping, i.e. a resonant process occurring as the magnetic field lines emerging from the disk are periodically compressed by the spiral wave. We show how it acts on a collisionless population of ions, trapped above the disk by the joint action of gravity and magnetic stresses. We discuss the efficiency of this mechanism in heating coronal particles and explaining observational evidence.