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Y.Y. Kovalev (NRAO, ASC LPI)
We have used visibility data obtained with the VLBA at 15 GHz during the period 1994--2002 to examine the most compact structure in 160 flat-spectrum extragalactic radio sources. With projected baselines out to 440 million wavelengths we are able to study source structure on angular scales as small as 0.12 mas. For about 40% of the sources, at least at one epoch, more than a half of the flux density is unresolved on the longest baselines, and have dimensions less than 0.12 mas. About a half of the sample has fringe visibilities greater than 0.5 Jy at the longest baselines. The most variable sources tend to have the most compact structure. For more than a half of the sample, at least at one epoch, the structure appears unresolved along a direction transverse to its extension. Typical lower limits to the brightness temperature are in the range 1011 to 1012 K but extends up to 1013 K apparently in excess of the inverse Compton limit of stationary synchrotron sources. The fringe visibility of the gamma-ray loud AGN show, on average, higher values than for gamma-ray quiet AGN and is more variable. This suggests a common emission mechanism at radio and gamma-ray wavelengths.
Longer baselines, such as shall be available from future space VLBI missions, will be needed to study the most compact high brightness temperature regions in these sources.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.