Intrinsic Brightness Temperatures of AGN Jets

We present a new method for studying the intrinsic brightness temperatures ofthe parsec-scale jet cores of Active Galactic Nuclei (AGN). Our method usesobserved superluminal motions and observed brightness temperatures for a largesample of AGN to constrain the characteristic intrinsic brightness temperatureof the sample as a whole. To study changes in intrinsic brightness temperature,we assume that the Doppler factors of individual jets are constant in time asjustified by their relatively small changes in observed flux density. We findthat in their median-low brightness temperature state, the sources in oursample have a narrow range of intrinsic brightness temperatures centered on acharacteristic temperature, T_int = 3 x 10^10 K, which is close to the valueexpected for equipartition, when the energy in the radiating particles equalsthe energy stored in the magnetic fields. However, in their maximum brightnessstate, we find that sources in our sample have a characteristic intrinsicbrightness temperature greater than 2 x 10^11 K, which is well in excess of theequipartition temperature. In this state, we estimate the energy in radiatingparticles exceeds the energy in the magnetic field by a factor of ~ 10^5. Wesuggest that the excess of particle energy when sources are in their maximumbrightness state is due to injection or acceleration of particles at the baseof the jet. Our results suggest that the common method of estimating jetDoppler factors by using a single measurement of observed brightnesstemperature and/or the assumption of equipartition may lead to large scatter orsystematic errors in the derived values.Comment: 4 pages, 2 figures, Accepted to Appear in ApJ Letter