Total flux density variations in extragalactic radio sources. II. Determining the limiting brightness temperature for synchrotron sources

The maximum intrinsic brightness temperature T-b,T-lim for powerful synchro tron-emitting radio sources is usually assumed to be approximate to 10(12) K, limited by the inverse Compton catastrophe. A lower value of approximate to 5 x 10(10) K, based on the equipartition brightness temperature, has be en suggested by Readhead on the basis of T-b,T-obs distributions derived fr om VLBI observations. We present two new methods for estimating T-b,T-lim i n extragalactic radio sources by using total flux density variations. A rea sonable estimate of the value of T-b,T-lim for a source can be obtained by comparing the Doppler boosting factors derived from total flux density vari ations at 22 and 37 GHz with traditional estimates based on the radio and s ynchrotron self-Compton (SSC) X-ray fluxes. Another independent estimate of T-b,T-lim is obtained by comparing the brightness temperatures derived fro m variability data with the values calculated from VLBI observations. Using several data sets, we find that both methods yield a value of less than or equal to 10(11) K, in accordance with the equipartition brightness tempera ture limit proposed by Readhead.