A Summary of Extreme Scattering Events and a Descriptive Model

An ongoing monitoring program of compact extragalactic radio sources over the past 11 years has revealed many instances of unusual variations that cannot be explained using the standard models for intrinsic variability or normal refractive interstellar scintillation. The observations included simultaneous daily measurements at 2.695 and 8.085 GHz of 40 to 150 sources using the Green Bank interferometer over the period 1979-1989. Approximately 330 sources were also monitored with the NRAO 300 foot radio telescope at 3.25 and 4.675 GHz over the 6 month period 1987 December to 1988 May. In total, 594 source-years of observations have been obtained, of which 4.8 yr constitutes the total time span of unusual variability. The unusual variations, denoted as extreme scattering events (ESEs), are summarized here. The most striking property of ESEs is their frequency dependence, in which departures from the quiescent flux density (by as much as 50% at 2.7 GHz) have no counterpart at 8.1 GHz. The single exception occurred in the 8.1 GHz light curve of 0954+658, which showed spiky variations during the 2.7 GHz event. To date, 10 ESEs have been tentatively identified in the 2.7 GHz light curves of nine quasars. These ESEs appear to occur preferentially near loops in the Galactic foreground brightness distribution. A simple model based on diffraction or refraction through Galactic regions of unusually high electron density turbulence is presented.