Intrinsic Brightness Temperatures of Compact Radio Jets as a Function of
  Frequency
  We present results of our investigation of the radio intrinsic brightness
temperatures of compact radio jets. The intrinsic brightness temperatures of
about 100 compact radio jets at 2, 5, 8, 15, and 86 GHz are estimated based on
large VLBI surveys conducted in 2001-2003 (or in 1996 for the 5 GHz sample).
The multi-frequency intrinsic brightness temperatures of the sample of jets are
determined by a statistical method relating the observed brightness
temperatures with the maximal apparent jet speeds, assuming one representative
intrinsic brightness temperature for a sample of jets at each observing
frequency. By investigating the observed brightness temperatures at 15 GHz in
multiple epochs, we found that the determination of the intrinsic brightness
temperature for our sample is affected by the flux density variability of
individual jets at time scales of a few years. This implies that it is
important to use contemporaneous VLBI observations for the multi-frequency
analysis of intrinsic brightness temperatures. Since our analysis is based on
the VLBI observations conducted in 2001-2003, the results are not strongly
affected by the flux density variability. We found that the intrinsic
brightness temperature $T_{\rm 0}$ increases as $T_{\rm 0}\propto\nu_{\rm
obs}^{\xi}$ with $\xi=0.7$ below a critical frequency $\nu_{\rm c}\approx9 {\rm
GHz}$ where energy losses begin to dominate the emission. Above $\nu_{\rm c}$,
$T_{\rm 0}$ decreases with $\xi=-1.2$, supporting for the decelerating jet
model or particle cascade model. We also found that the peak value of $T_{\rm
0}\approx3.4\times10^{10}$ K is close to the equipartition temperature,
implying that the VLBI cores observable at 2-86 GHz may be representing jet
regions where the magnetic field energy dominates the total energy in jets.
