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

The maximum intrinsic brightness temperature T b , lim for powerful synchrotron-emitting radio sources is usually assumed to be ≈10 12 K, limited by the inverse Compton catastrophe. A lower value of ≈5 × 10 10 K, based on the equipartition brightness temperature, has been suggested by Readhead on the basis of T b , obs distributions derived from VLBI observations. We present two new methods for estimating T b , lim in extragalactic radio sources by using total flux density variations. A reasonable estimate of the value of T b , lim for a source can be obtained by comparing the Doppler boosting factors derived from total flux density variations at 22 and 37 GHz with traditional estimates based on the radio and synchrotron self-Compton (SSC) X-ray fluxes. Another independent estimate of T b , lim is obtained by comparing the brightness temperatures derived from variability data with the values calculated from VLBI observations. Using several data sets, we find that both methods yield a value of ≤10 11 K, in accordance with the equipartition brightness temperature limit proposed by Readhead.