An intergalactic origin for the low-frequency flux variations of extragalactic radio sources

Abstract

Measurements of the X-ray background, absorption-line spectra of quasars and the depolarization of their radio emission indicate that the intergalactic space is filled with clouds of thermal plasma. These clouds are likely to contain density inhomogeneities on a wide range of physical scales, like the interstellar medium (ISM) of our own galaxy4. Here I discuss a new possibility, viz. that the flux-density variations of compact extragalactic radio sources observed at low frequencies (v <1 GHz) could substantially arise from refractive focusing and defocusing of their emission by intervening intergalactic plasma inhomogeneities (on parsec scale), possibly associated with the cooling flows in the cores of clusters of galaxies. The observed time-scales of a few years for the low-frequency variability (LFV) of bright compact radio sources can be understood by considering that such sources are parts of relativistic jets and usually appear to separate from the parent galactic nucleus at superluminal velocities. Consequently, the line of sight to such a source is expected to sweep past any distant intervening plasma cloud typically at apparent speeds >c. Observational relevance of the resulting 'superluminal refractive scintillations', as proposed here, is briefly discussed.