Total Flux Density Variations in Extragalactic Radio Sources. II. Determining the Limiting Brightness Temperature for Synchrotron Sources

Abstract

The maximum intrinsic brightness temperature for powerful synchrotron-emitting radio sources T b,lim is usually assumed to be B1012 K, limited by the inverse Compton catastrophe. A lower value of B5 ] 1010 K, based on the equipartition brightness temperature, has been suggested by Readhead on the basis of distributions derived from VLBI observations. We present two new methods for esti-T b,obs mating in extragalactic radio sources by using total Ñux density variations. A reasonable estimate T b,lim of the value of for a source can be obtained by comparing the Doppler boosting factors derived T b,lim from total Ñux density variations at 22 and 37 GHz with traditional estimates based on the radio and synchrotron self-Compton (SSC) X-ray Ñuxes. Another independent estimate of is obtained by T b,lim comparing the brightness temperatures derived from variability data with the values calculated from VLBI observations. Using several data sets, we Ðnd that both methods yield a value of ¹1011 K, in accordance with the equipartition brightness temperature limit proposed by Readhead.