Variability , Brightness Temperature , Superluminal Motion , Doppler Boosting , and Related Issues

We review the observations of rapid flux density variations in compact radio sources, and discuss the inverse Compton limit to the maximum brightness temperature of incoherent synchrotron sources in comparison with recent VLBA observations. The apparent agreement of the theoretical brightness temperature limit due to inverse Compton cooling and the brightness temperatures observed by early VLBI observations appears to have been fortuitous. VLBA observations have greatly improved the quality of the data, but many of the early issues remain unresolved. 1. Variability Issues Starting in the mid 1960s, it was becoming increasingly clear that many flat spectrum radio sources exhibited very rapid flux density variations (e.g., Sholomitskii 1965; Dent 1965; Pauliny-Toth & Kellermann 1968). This presented serious problems for conventional synchrotron models as it appeared to require a huge energy content and an excessive amount of inverse Compton cooling. 1.1. The Saga of 3C 120 One of most variable sources observed in the early years was PKS 0430+05 (Day et al. 1966) also known as NRAO182 (Pauliny-Toth et al. 1966), which is now widely, but incorrectly, known as 3C 120. The misunderstanding about the nomenclature of this source goes back to the program of flux density and variability observations made with the newly completed 140-ft radio telescope at 10, 6, and 2 cm. The observing list for these programs was created by combining the source lists which Ivan Pauliny-Toth and I had separately made from our previous work at the 300 ft (mostly norther sources) and Parkes telescopes (mostly southern sources) respectively. One source was common to our two lists, but was called NRAO182 by Pauliny-Toth and PKS0430+05 by myself. Thinking that such a strong centimeter source should have been included in the better known low frequency catalogs, we searched in vain for a source at the same or nearby position in the 3CR catalog (Bennett et al. 1962) Our inspection of the original confusion limited 3C Catalog was, at best, confusing. The position (B1950) given in the NRAO VLA Calibrator Manual is RA = 043031. 6 and Dec = +051459. 6. The position of 3C 120 in the original 3C Catalog is RA = 042932 ± 4 and Dec = +0155 ± 8 which is more than 3 degrees away in declination. However, due to noise and confusion, the positions of some sources in the 3C Catalog can be in error by one or more