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The rapid variations in flux of the radio source 0917+624 are analyzed to determine the contribution of refractive interstellar scintillations (RISS). An analysis is presented of simultaneous recordings of flux at wavelengths 3.6cm, 6cm, 11cm and 20cm made at Bonn with the 100 m telescope, (provided by A. Witzel, MPIfR). The variations are characterized by a time scale that increases from 3 hrs at 3.6cm to 18 hrs at 20cm. When the 3.6cm variations are smoothed with a time constant equivalent to the characteristic time at 6cm, the result is a series that is 90\% correlated with the actual 6cm variations. Similar relationships exist between the 6cm and 11cm variations. Such behavior is not predicted by any of the intrinsic models for the variations, but is expected for RISS, in which the source diameter increases linearly with wavelength and is also greater than the scattered diameter. The 20cm data show a somewhat different behavior. This could either be due to intrinsic source variations or to RISS from a 20cm source component substantially different from that at shorter wavelengths. The amplitude of the variations is also analysed versus wavelength and a model for the RISS and the source structure is proposed. The diameter at 6cm of the compact component is estimated to be between 15 and 100 microarcseconds.
The conclusion is that the major fraction of the variance in the cm wavelength flux of 0917+624 is due to RISS. Previous interpretations of "intraday" variations have relied entirely on intrinsic mechanisms, and have required extreme source models. Whereas the present study does not rule out intrinsic variations on times of a less than a day, it demonstrates that very compact sources exhibit rapid RISS, that must be included for a proper interpretation. The study also shows how these variations can help to refine our understsanding of RISS and so of the microstructure in the interstellar plasma.
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