SIMBAD references

The rapid variations in flux density of the intraday variable radio source 0917+624 are analysed to determine the contribution of interstellar scintillations (ISS). Most previous interpretations have involved intrinsic variations of a very compact source requiring extremely high brightness temperatures or Doppler factors. An analysis is presented of simultaneous recordings of flux density at wavelengths 2cm, 3.6cm, 6cm, 11cm and 20cm. The variations are characterized by a time scale that increases from 2.5hrs at 2cm to 18hrs at 20cm. A high degree of correlation with no time offset is found between the slowly-varying components of the variations at 3.6cm, 6cm and 11cm. Whereas intrinsic models predict variations with a delay that increases with wavelength, an ISS model predicts the correlation as observed, under the condition that the source diameter increases with wavelength and is greater than the scattered diameter. A detailed ISS model for the scintillation is proposed, including a specific source structure and a 200pc distance to the scattering region. The brightness temperature is estimated to be 6x10¹²K, consistent with studies of superluminal sources. Though polarization variations are only approximately consistent with the model, the conclusion is that the major fraction of the variance in the cm wavelength flux density of 0917+624 is due to ISS. Whereas the present study does not rule out additional intrinsic variations on times of a less than a day, it demonstrates that very compact sources exhibit rapid ISS, which must be included for a proper interpretation.