Rapid variability of extragalactic radio sources

Thirty-one extragalactic radio sources have been observed at 2-4 hr intervals for 3-4 days at each of three epochs. The 15 compact sources in the sample appear to display two types of variability. One is probably weak flickering caused by refractive interstellar scintillation. The other is a larger amplitude variability, which is characterized by a narrow range of timescales of about 1-2 days. Three possible causes of the second type are discussed: (1) intrinsic variability; (2) scintillation from a very narrow disk component of the ISM; and (3) scintillation from clouds associated with a particular region of the ISM. It is suggested that the latter is the more likely explanation.

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