Jet Outbursts from Fast Accretion in a Disk with Zebra‐Stripe Magnetic Field

Abstract

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 events in a magnetized accretion disk of a massive black hole. Here we discuss the theory and simulation results for a time-dependent, axisymmetric disk accretion model, including the influence of an ordered magnetic field that reverses polarity as a function of radial distance in the disk. The accretion rate of the disk is coupled to the rate of angular momentum and energy outflow in magnetically driven jets originating from the ±z surfaces of the disk. The inward radial accretion speed in the disk (u) is the sum of the familiar viscous term and a magnetic term proportional to r3/2B2z/σ due to the jets, where Bz(r, t) is the field at the midplane threading the disk, and σ(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 Bz and σ. For general initial conditions, Bz and σ vary with r. Furthermore, Bz necessarily reverses polarity in order to conserve flux. As a result of the polarity reversals, the evolution of σ and Bz leads to the formation of inward facing shocks, where the radial derivatives of Bz and σ are very large. The shocks separate different annular regions of the disk threaded by positive and negative Bz. The kinetic luminosity in the jets is predominantly 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 outburst of an AGN and the associated creation of a new parsec-scale jet component. Also in this picture, the outburst corresponds to a reversal of polarity of Bz in the inner part of the disk. As a result of the jet's propagation and radial expansion, this polarity reversal becomes a polarity reversal of Bϕ as z varies across the jet component. Consequently, magnetic field annihilation in the jet component may be important, in particular, for accelerating the leptons to the high Lorentz factors needed to explain the observed synchrotron, synchrotron self-Compton, and inverse Compton radiation.