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
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.
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
are common. As the survey progresses,
a more complete census of the brightness temperature properties of
AGN cores will emerge.
to the University of Calgary Space VLBI Web Page.
Comments and inquiries to:
Russ Taylor: (email@example.com)
Bill Scott: (bill at ras.ucalgary.ca)