Possible mechanisms fall into two categories:
4.1. Processes involving synchrotron-type emission from relativistic particles
The prime requirement here is an acceleration mechanism capable of
channeling much of the energy into relativistic electrons.
Acceleration mechanisms, though ill-understood, are known to be
widespread, even in astrophysical contexts where conditions are far
less extreme than in quasars. For instance it appears that
decelerating supernova remnants such as Cass A have converted a few
percent of their kinetic energy into cosmic ray particles. The most
likely mechanisms involve either Fermi-type processes in shock fronts,
or else field amplification via Rayleigh-Taylor instabilities which
then permits efficient acceleration at neutral sheets. The velocities
involved in supernova remnants are
10-2
c. When non-laminar
accretion occurs onto a massive black hole, one expects shock waves
near the hole (where most of the energy is released) to involve
velocities larger than this; so whatever process operates in supernova
remnants should occur even more efficiently. This is the most
"conservative" class of acceleration mechanism. If relativistic blast
waves [36]
occur, then electrons can attain
1 by a "one-step"
process, without the need for stochastic or Fermi-type processes
across or behind the shock.
Some authors have considered magnetic flares in massive accretion disks - it is certainly plausible that much of the energy emitted near the hole, where orbital velocities may be ~ c/2, should go directly into relativistic particles, and eventually be radiated non-thermally.
A more exotic possibility has been explored by Blandford and Znajek
[35],
who have investigated the electromagnetic effects close to a
black hole which accretes from a surrounding disk of plasma with a
strong magnetic field. Even though the MHD approximation may hold in
the disk (so that there is no E-field in a frame moving with the
rotating matter), there may be insufficient plasma along the axis to
short out an induced E-field. This E-field is limited by a vacuum
breakdown whereby a single energetic electron (or photon) could
initiate a shower of e+-e- pairs. This breakdown
may resemble the
Sturrock-Ruderman-Sutherland curvature radiation process originally
proposed in the pulsar context, or could be initiated by Compton
scattering of soft photons by high-energy electrons (and resulting
pair production). For quasar parameters, one gets
e+-e- pairs with
Lorentz factor
106. This process
in principle extracts rotational
energy from a Kerr hole. This does not make much impact on the energy
budget - a bigger contribution to comes from the gravitational binding
energy of the infalling matter - but the hole's contribution is more
likely to go directly into the "low entropy" form of ultrarelativistic
particles which escape preferentially along the rotation axis. It may
be relevant to note also that bulk expansion of an
e+-e- plasma can
attain higher Lorentz factors than an ordinary plasma, because in the
latter case the ions provide greater rest mass. This is relevant to
the interpretation of "superlight" velocities in radio components,
discussed elsewhere in these proceedings.