2006ApJS..165..439R

From 1979 to 1996 the Green Bank Interferometer was used by the Naval Research Laboratory to monitor the flux density from 146 compact radio sources at frequencies near 2 and 8 GHz. We filter the ``light curves'' to separate intrinsic variations on times of a year or more from more rapid interstellar scintillation (ISS) on times of 5-50 days. Whereas the intrinsic variation at 2 GHz is similar to that at 8 GHz (although diminished in amplitude), the ISS variation is much stronger at 2 than at 8 GHz. We characterize the ISS variation by an rms amplitude and a timescale and examine the statistics of these parameters for the 121 sources with significant ISS at 2 GHz. We model the scintillations using the NE2001 Galactic electron model assuming the sources are brightness-limited. We find the observed rms amplitude to be in general agreement with the model, provided that the compact components of the sources have about 50% of their flux density in a component with maximum brightness temperatures 10¹¹-10¹² K. Thus, our results are consistent with centimeter-wavelength VLBI studies of compact active galactic nuclei, in that the maximum brightness temperatures found are consistent with the inverse synchrotron limit at 3x10¹¹ K, boosted in jet configurations by Doppler factors up to about 20. The average of the observed 2 GHz ISS timescales is in reasonable agreement with the model at Galactic latitudes above about 10°. At lower latitudes the observed timescales are too fast, suggesting that the transverse velocity increases beyond about 1 kpc, which may be due to differential Galactic rotation.