Intrinsic Brightness Temperature of Compact Radio Sources at 86 GHz
  We present results on the intrinsic brightness temperature of a sample of
compact radio sources observed at 86 GHz using the Global Millimeter VLBI
Array. We use the observed brightness temperatures at 86 GHz and the observed
superluminal motions at 15 GHz for the sample in order to constrain the
characteristic intrinsic brightness temperature of the sample. With a
statistical method for studying the intrinsic brightness temperatures of
innermost jet cores of compact radio sources, assuming that all sources have
the same intrinsic brightness temperature and the viewing angles of their jets
are around the critical value for the maximal apparent speed, we find that
sources in the sample have a characteristic intrinsic brightness temperature,
$T_{\rm 0} = 4.8^{+2.6}_{-1.5}\times 10^{9}$ K, which is lower than the
equipartition temperature for the condition that the particle energy equals to
the magnetic field energy. Our results suggest that the VLBI cores seen at 86
GHz may be representing a jet region where the magnetic field energy dominates
the total energy in the jet.
