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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. Access through your institution Buy or subscribe This is a preview of subscription content, access via your institution ACCESS OPTIONS
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institutional subscriptions * Read our FAQs * Contact customer support SIMILAR CONTENT BEING VIEWED BY OTHERS GAMMA-RAY FLARES FROM RELATIVISTIC MAGNETIC RECONNECTION IN THE JET OF THE
QUASAR 3C 279 Article Open access 21 August 2020 LIGHT BENDING AND X-RAY ECHOES FROM BEHIND A SUPERMASSIVE BLACK HOLE Article 28 July 2021 A RADIO TRANSIENT WITH UNUSUALLY SLOW PERIODIC
EMISSION Article 26 January 2022 REFERENCES * 1. Fiedler, R. L., Dennison, B., Johnston, K. J. & Hewish, A. Nature (submitted). 2. Romani, R. W., Narayan, R. & Blandford, R. D. Mon.
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AUTHORS AND AFFILIATIONS * Theoretical Astrophysics 130–33, California Institute of Technology, Pasadena, California, 91125, USA Roger W. Romani & Roger D. Blandford * Astronomy
Department and NAIC, Cornell University, Ithaca, New York, 14853, USA James M. Cordes Authors * Roger W. Romani View author publications You can also search for this author inPubMed Google
Scholar * Roger D. Blandford View author publications You can also search for this author inPubMed Google Scholar * James M. Cordes View author publications You can also search for this
author inPubMed Google Scholar RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Romani, R., Blandford, R. & Cordes, J. Radio caustics from localized
interstellar medium plasma structures. _Nature_ 328, 324–326 (1987). https://doi.org/10.1038/328324a0 Download citation * Received: 13 March 1987 * Accepted: 07 May 1987 * Issue Date: 23
July 1987 * DOI: https://doi.org/10.1038/328324a0 SHARE THIS ARTICLE Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is
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