3.4. Average Hot-Spot Advance Speeds from Spectral Measurements
Estimates of 
h can be
made by measuring the effects of synchrotron losses in the
lobes of FRII sources. If the synchrotron electrons are accelerated at
the hot-spot and
stream away to form the lobe, then the curvature of the spectrum at a
given point in
the lobe is determined by the subsequent energy losses. The assumptions
most usually
made are that the losses are due entirely to synchrotron radiation and
Inverse-Compton
scattering off the microwave background radiation, that the input energy
distribution
is a power law consistent with the low-frequency spectral index of the
hot-spot and
that the electrons radiate in a uniform magnetic field whose strength is
given by the equipartition condition. Any of these assumptions may be
incorrect.
The most extensive sets of multifrequency measurements are those of
Alexander & Leahy (1987)
and Liu, Pooley &
Riley (1992).
Figure 3 (taken from the latter reference)
shows a plot of mean advance speed against radio luminosity for the
combined samples. The trend is for the velocities to increase from
h ~ 0.01c at
luminosities around the
FRI/FRII division to h
~ 0.15c at very high luminosities. The velocities are sensitive to
the assumptions used to derive the magnetic field strength, but even if
the electrons lose
energy purely by Inverse Compton scattering off the microwave background
radiation, the velocities can be as high as 0.07c.
|
Figure 3. A plot of mean hot-spot advance
speed, |