Annu. Rev. Astron. Astrophys. 1997. 35: 607-36
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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 betaapp in the range 1-5 h-1 c occur with roughly equal frequency (Figure 7), and in particular, higher values up to betaapp = 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 beta 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

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.

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