Annu. Rev. Astron. Astrophys. 1981. 19: 373-410 Copyright © 1981 by Annual Reviews. All rights reserved

6. STRUCTURAL VARIATIONS

The compact radio sources are nearly all variable, and since they have complex brightness distributions, it is expected that in general the brightness distribution will vary with time. The detailed manner in which this occurs is clearly important as it may relate directly to the way in which relativistic particles gain and lose energy, and perhaps to the ultimate source of energy itself. The major questions are: Do the separate flux density outbursts occur in the same part of the source or in different locations? If the latter, does the location change randomly or systematically? Do the radiating regions move, and if so, how?

At present, repeated observations exist for only a few of the stronger sources. Although, for some of these, the observations extend over a decade of time, only since the late 1970s have the data been sufficicnt to describe the variations in any detail. No simple picture has emerged from a complex pattern of intensity changes combined with apparent component motions. Among the brightest sources, four (3C 120, 3C 273, 3C 279, and 3C 345) show evidence for apparent faster-than-light component motion with 5 v/c 45 (Cohen et al. 1977, Pauliny-Toth et al. 1981). For another source, 3C 454.3, there are limited data from 1969 during a strong radio outburst which indicate superluminal motion, with v/c ~ 3 (Gubbay et al. 1977). Two other strong sources, 3C 84 (NGC 1275) and 4C39.25 (0923 + 39), show no component motion at all, while for another, 2134 + 00, there is no evidence for motion, but the data are very limited (Schilizzi et al. 1975, Pauliny-Toth et al. 1981).

Because the flux densities of the compact sources vary, often by a factor of two or more, there is no unique listing of then brightest sources. BL Lac and 0235 + 16 are at times included among the brightest 10 sources, and at times 3C 120 is not included. The limited data available for BL Lac can be interpreted in terms of flux density variations in fixed components (Clark et al. 1973, Kellermann et al. 1977b). There were unfortunately no VLBI data during the 1975 outburst in 0235 + 16, but the short time scale and absence of interstellar scintillations suggest superluminal motion in 0235 + 16 as well (Scheuer 1976). Although it is difficult to make precise statements about the fraction of sources that show superluminal motion, roughly half of the stronger sources, at least, appear to show this effect, and this has important implications for interpretations that appeal to specialized geometries.