Copyright © 1997 by Annual Reviews. All rights reserved
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| Annu. Rev. Astron. Astrophys. 1997. 35:
607-36 Copyright © 1997 by Annual Reviews. All rights reserved |
3.1. Apparent Superluminal Motion
Statistical results from the study of motions in large samples of
superluminal sources have been discussed by
Ghisellini et al (1993),
Vermeulen & Cohen
(1994),
Vermeulen (1995),
for example. These studies aim at developing population models that test
for the presence of a distribution in lieu of a constant value of Lorentz
factor, pattern speeds that differ from fluid speeds, jet bending, and
accelerations. For a sample of 81 flat-spectrum objects,
Vermeulen (1995)
found no evidence for intrinsically different populations of galaxies, BL
Lac objects, and quasars, which is in contrast to other reports (see
Gabuzda 1995).
Apparent velocities
app
in the range 1-5 h-1 c occur with roughly equal
frequency (Figure 7), and in particular, higher
values up to
app
= 10 h-1 are rarer than assumed from earlier studies
of superluminal sources (see Figure 7).
This velocity distribution can be reproduced by assuming a wide range of
Lorentz factors. Finally, a rising upper envelope to the
app distribution appears to exist when plotted as a function of
5-GHz luminosity. This is consistent with the high-luminosity sources in
the sample existing as strongly Doppler-beamed members of a parent
population with much lower intrinsic luminosity.
 |
Figure 7. The observed apparent velocity distribution for 81 objects in a homogeneous flat-spectrum sample is illustrated, showing that higher values are rare, compared with earlier work and predictions for beamed samples (from Vermeulen 1995). |
In the majority of cases, the evidence for apparent superluminal motion is based on few observations, and accordingly, such statistical studies usually assume that the motion measured for a particular superluminal source component is along ballistic trajectories at constant speed. Detailed monitoring studies have shown that in a given source, not only can different components have different speeds, but accelerations and decelerations are present in several sources. Stationary features have been observed in a few instances. In addition, rarely are the reported motions along straight ballistic trajectories: Kinks and bends are frequently seen, and in a small number of cases, complicated curved trajectories have been determined. Note that such curvature sometimes is measured not directly for a given component, but indirectly by tracing several components, e.g. along a jet that might be characterized by a well-defined ridge line or time-averaged mean jet axis. The curvature and nonlinear motion along bent trajectories that is seen so often is perhaps the strongest evidence against the existence of truly moving plasmons as the physical nature of superluminally moving components.