Abstract
The flux variations of extragalactic radio sources at low radio frequencies have proved very difficult to explain as changes in the luminosity of a simple synchrotron source. To circumvent this problem, several more complicated source models, such as those involving relativistic expansion of the source or coherent radiation mechanisms, have been devised. However, it has been suggested recently that the observed flux changes may be the result of a focusing effect of irregularities in the local interstellar medium. Here we present evidence which suggests that, in accordance with this explanation, low frequency variability is a more common feature of sources at low galactic latitudes. In the sample of radio sources monitored over a period of five years at Bologna, the mean galactic latitude of the variable sources is smaller than that of the non-variables to an extent that would occur with a chance of only 2% were the two groups drawn from the same parent distribution.
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Cawthorne, T., Rickett, B. Low frequency variability and interstellar focusing. Nature 315, 40–42 (1985). https://doi.org/10.1038/315040a0
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DOI: https://doi.org/10.1038/315040a0
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