Interstellar scintillation observations of 146 extragalactic radio sources

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 10(11)-10(12) 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 3 x 10(11) 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 degrees. 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.