|
JET OUTBURSTS
FROM FAST ACCRETION IN A DISK WITH ZEBRA-STRIPE MAGNETIC FIELD
Authors:
R.V.E. Lovelace, W.I. Newman, and M.M. Romanova
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 (AGN) 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 which 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 µ r3/2 Bz2 /
s
due to the jets, where Bz(r,t) is the field at the midplane threading the
disk,
and s(r,t) is the
disk's surface mass density. We consider conditions where the magnetic term is dominant
and derive coupled nonlinear equations for the evolution of Bz
and s.
For general initial conditions, Bz and s vary with r.
Furthermore, Bz necessarily reverses polarity in order
to conserve flux. As a result of the polarity reversals, the evolution of s
and Bz leads to the formation of inward facing shocks where the radial derivatives of
|| Bz
|| and s are very large. The shocks separate different annular regions of
the disk threaded by positive and negative Bz. The 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 luminosity. This spike we interpret as an outburst of an AGN and the associated
creation of a new pc-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 Bj 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.
[return]
|
