Variability and VLBI Observations of Extragalactic Radio Sources

Abstract

When Maarten Schmidt (1963) recognized that the previously unidentified lines in what had thought to be radio stars were nothing else but highly redshifted Baimer lines, it became clear that these quasi-stellar objects were in fact distant sources with enormous energy outputs. Only two years later, Sholomitskii (1965) detected variations in the radio source CTA 102 on the time scale of a few months; when it was identified with a quasar at redshift z = 1.037, the conclusion that the most energetic objects in the universe had dimensions of only a few light months seemed inevitable. From the spectra and the observed polarization characteristics of quasars, it is concluded that their radio emission is due to incoherent synchrotron radiation. In fact, it was shown that the spectral behavior of the variations observed in a number of extragaiactic radio sources could be explained by a simple model based on the emission of an expanding homogeneous synchrotron source (Kellermann and Pauliny-Toth 1968). It is important to note here that the surface brightness of a synchrotron source cannot exceed a certain critical value: A source with a brightness temperature T _B > 10¹² K would cool down to this value in a matter of seconds due to second order inverse Compton scattering (Kellermann and Pauliny-Toth 1969).The source sizes inferred from the mentioned variability observations were indeed so small that many sources seemed to violate the inverse Compton limit. It was Rees (1966) who pointed out that this discrepancy could be resolved by postulating highly relativistic motion of the synchrotron-emitting plasma.