The Synchro-Compton Limit of the Brightness Temperature of Nonstationary Radio Sources

The brightness temperature of synchrotron emission from nonstationary radio sources during the cooling down by the inverse Compton effect is calculated. It is shown that brightness temperatures as high as 5 x 10 exp 15 K at 1 GHz are allowed during the first day after injection of relativistic electrons of sufficiently high energy. This is about four orders of magnitude higher than the canonical synchro-Compton limit introduced by Kellermann and Pauliny-Toth (1969) for stationary radio sources. A stationary situation with the in situ first-order Fermi acceleration will give a brightness temperature of about 10 exp 15 K at 1 GHz due to the compensation of the inverse Compton losses by particle acceleration. The high brightness temperature effect is most pronounced at low frequencies and is proposed as the explanation of the LF variability phenomenon. Strong high-energy emission is predicted during phases of high brightness temperature.