JET OUTBURSTS FROM FAST ACCRETION IN A DISK WITH ZEBRA-STRIPE MAGNETIC-FIELD

The optical, X-ray, and gamma-ray outbursts, as well as the associated formation of relativistically moving components of parsec-scale jets of some active galactic nuclei (AGNs) are interpreted as dynamical eve nts in a magnetized accretion disk of a massive black hole. Here we di scuss the theory and simulation results for a time-dependent, axisymme tric disk accretion model, including the influence of an ordered magne tic field that reverses polarity as a function of radial distance in t he disk. The accretion rate of the disk is coupled to the rate of angu lar momentum and energy outflow in magnetically driven jets originatin g from the +/-z surfaces of the disk. The inward radial accretion spee d in the disk (u) is the sum of the familiar viscous term and a magnet ic term proportional to r(3/2)B(x)(2)/sigma due to the jets, where B-z (r, t) is the field at the midplane threading the disk, and sigma(r, t ) is the disk's surface mass density. We consider conditions where the magnetic term is dominant, and we derive coupled nonlinear equations for the evolution of B-z and sigma. For general initial conditions, B- z and sigma vary with r. Furthermore, B-z necessarily reverses polarit y in order to conserve flux. As a result of the polarity reversals, th e evolution of sigma and B-z leads to the formation of inward facing s hocks, where the radial derivatives of B-z and sigma are very large. T he shocks separate different annular regions of the disk threaded by p ositive and negative B-z. The kinetic luminosity in the jets is predom inantly from the innermost part of the disk. Consequently, the passage of a shock through the inner edge of the disk gives a strong, narrow spike in the jet kinetic luminosity. We interpret this spike as an out burst of an AGN and the associated creation of a new parsec-scale jet component. Also in this picture, the outburst corresponds to a reversa l of polarity of B-z in the inner part of the disk. As a result of the jet's propagation and radial expansion, this polarity reversal become s a polarity reversal of B-phi as z varies across the jet component. C onsequently, magnetic field annihilation in the jet component may be i mportant, in particular, for accelerating the leptons to the high Lore ntz factors needed to explain the observed synchrotron, synchrotron se lf-Compton, and inverse Compton radiation.