STATISTICS OF INTRADAY VARIABILITY IN EXTRAGALACTIC RADIO-SOURCES

Intraday variations with amplitudes ranging up to approximately 25% have been found in several compact extragalactic radio sources. Here we present the results of a search for such variations in two complete samples of flat-spectrum sources. Low-level fluctuations are found in virtually all sources that have a compact VLBI structure; this result is compatible with the expectation that the influence of refractive interstellar scintillation should be detectable on the 1-2% level. Larger variations are observed in about 25% of the flat-spectrum objects. The statistical data on these sources do not rule out interstellar scintillation as a possible origin of the variability; they are also compatible with an intrinsic explanation as suggested by more detailed investigations of individual sources, however.