Brightness Temperature Determination

In those cases when the number of participating ground telescopes is inadequate to provide enough information to synthesize an image, information about the AGN's core can still be obtained by determination of the core brightness temperature. Model-fitting of visibilities directly allows one to place an upper limit on the size of the AGN core. This, coupled with a measurement of source intensity, gives one a lower bound on the brightness temperature.

For the presumed radiation process, incoherent synchrotron radiation, there is a theoretical upper limit on the brightness temperature of approximately 1012K. At these high temperatures, the electons generating the radiation start to lose their energy catastrophically to the radiation field via electon-photon scattering (Inverse Compton scattering). Brightness temperatures of a few factors in excess of this limit are possible if one assumes the "intrinsic brightness temperature" of the AGN has been enhanced by relativistic Doppler beaming effects.

It is an unfortunate coincidence, however, that for the strongest AGN sources the maximum lower limit that can be placed on the brightness temperature, using only Earth-based VLBI, is ~ 1012K. Since the lower brightness temperature limit is determined by the length of the maximum baseline, independent of frequency, shorter wavelength observations will not improve the situation. Space-ground baselines are needed.

Initial brightness temperature measurements for VSOP Survey sources have confirmed that brightness temperatures in excess of 1012K are common. As the survey progresses, a more complete census of the brightness temperature properties of AGN cores will emerge.

Return to the University of Calgary Space VLBI Web Page.

Comments and inquiries to:
Russ Taylor: (
Bill Scott: (bill at