Radio caustics from localized interstellar medium plasma structures

Abstract

In a study of, 36 extragalactic radio sources observed over 7 years with the Green Bank interferometer at 2.7 GHz and 8.1 GHz, Fiedler et al.¹ have detected unusual variations in the light curves of several sources. The most dramatic event, found in the flux record of the quasar 0954 + 658, showed large modulations at both frequencies. At 8.1 GHz, there are four spikes over a period ∼80 days in each of which the intensity increases roughly threefold. At 2.7 CfHz, two broad maxima of width ∼30 days bracket the high-frequency event, with a total duration of ∼120 days. At least two other sources show similar 2.7-GHz variations. Fiedler et al. argue cogently that these variations are unlikely to be intrinsic to the sources and that refractive scintillation in the interstellar medium is probably responsible. They postulate the existence of large-scale (10¹³–10¹⁴ cm) localized plasma density inhomo-geneities in the interstellar medium (ISM) to explain these observations. Independent evidence for strong local fluctuations in the ISM electron density on these scales comes froin the unusually large amplitude of low-frequency quasar variation², the presence of periodicities in pulsar dynamic scintillation spectra³ and non-gaussian spikes in pulsar light curves⁴. All three phenomena have been interpreted as multiple imaging and focusing by large-scale refractors. Here we present a more detailed interpretation of the event in 0954 + 658, propose possible sites for the refracting clouds and suggest some future observations.