The asymmetry coefficient for interstellar scintillation of extragalactic radio sources

Comparing the asymmetry coefficients γ and scintillation indices m for observed time variations of the intensity of the radiation of extragalactic sources and the predictions of theoretical models is a good test of the nature of the observed variations. Such comparisons can be used to determine whether flux density variations are due to scintillation in the interstellar medium or are intrinsic to the source. In the former case, they can be used to estimate the fraction of the total flux contributed by the compact component (core) whose flux density variations are caused by inhomogeneities in the interstellar plasma. Results for the radio sources PKS 0405-385, B0917+624, PKS 1257-336, and J1819+3845 demonstrate that the scintillating component in these objects makes up from 50 to 100% of the total flux, and that the intrinsic angular sizes of the sources at 5 GHz are 10–40 microarcseconds. The characteristics of the medium giving rise to the scintillations are presented.