Annu. Rev. Astron. Astrophys. 1988. 36:
539-598
Copyright © 1998 by . All rights reserved |

**3.4. Observable Quantities**

POWER In the Blandford & Payne model, the ratio of jet and disk luminosities is

(10) |

where *r*_{0 max} and *r*_{0 min} are the
outer and inner disk radii and
is an efficiency
factor of transformation of binding energy at *r*_{0 min}
into jet power. For the estimated parameters, this ratio is always
<< 1; i.e.
only a small fraction of the accreted mass has to go into the wind. Only
for relatively high-mass discharge onto the jet,
_{j} ~ 0.1
*M*_{} / year
~ 0.1 _{d}, can
the power in the jet reach *L*_{j}
~ 10^{46} erg s^{-1};
this makes it difficult to explain the energetics of strong sources (see
Section 2), especially when taking into
account that only a small fraction can be transformed into
radiation. Analytic results are confirmed substantially
by the numerical simulations we discussed above. Hydromagnetic jets appear
to be a very efficient way to extract energy from accretion disks, but
perhaps a relativistic treatment is needed to fit the parameters well.

In this respect, unipolar inductor-type models are more promising. Camenzind (1998) showed that the Poynting flux of a rotating axisymmetric Kerr black hole magnetosphere can carry a magnetic luminosity:

(11) |

where _{h}
is the rotational velocity at the Kerr horizon and *I*_{h}
the total current and
_{h} the
magnetic flux, respectively, integrated over the horizon.

While the understanding of the jet dynamics
has progressed substantially, the problem of powering jets and enabling them
to emit nonthermal radiation with luminosities up to ~ 10^{47}
erg s^{-1} is still far from being solved.

ASYMPTOTIC BULK VELOCITY
In most jets, asymptotic bulk
velocity is estimated indirectly through proper motions at VLBI resolution
or through advancement velocities of extended structures or required Doppler
beaming. This bulk velocity has to be supersonic and, in the inner portion
of jets, relativistic, with Lorentz factors up to
_{bulk}
~ 20. In general, the asymptotic velocity of outflows in hydromagnetic
models is a few times the Keplerian velocity at the base of the flow,
which corresponds to super-Alfvènic and relativistic velocities and
allows Lorentz factors that are as large as needed,
_{bulk}
~ 10-20.
Kudoh & Shibata
(1995)
have investigated 1.5-D steady MHD winds from accretion disks, including
thermal effects, and they obtained a relation between the jet mass flux and
the magnetic energy of the disk. Their calculations confirm the above
results for the terminal velocity with a weak dependence on the magnetic
energy of the disk,
*v*_{}
*B*^{1/3}.