| The existence of extremely high observed brightness temperatures in some AGN cores is one of the most important
discoveries made by VLBI. The Pearson-Readhead Survey from Space (Lister et al. 2001) conducted with VSOP
measured brightness temperature lower limits for some sources of a few times 10 12 K, requiring high Doppler factors
(Æ > 30) if these sources are in equipartition. High brightness temperatures can be measured by using either long
baselines or high-sensitivity observations. Observations using baselines from the VLBA to the GBT should be able to
measure brightness temperatures four times higher than those measured by VSOP (see justi cation for derivation).
We propose to observe the three sources from Lister et al. (2001) that had measured brightness temperature lower
limits greater than 2 10 12 K, using the VLBA+GBT+EB. These observations could increase the measured lower
limits by a factor of four or more, which would push these brightness temperature measurements into physically
interesting regions that challenge models of radio cores. |
