Rapid polarization variations at 20 cm in 0917+624

The intraday variations (IDV hereafter) of the total and the polarized flux density observed at 20 cm in 0917+624 tin May 1989) are analysed. It is shown that the IDV can be interpreted in terms of refractive interstellar scintillation. The observed variations are dominated by one scintillating component with a timescale tau approximate to 0.6 days and a scintillation index m approximate to 0.04. In addition, one more scintillating component with a shorter timescale and a smaller scintillation index is needed to improve the fit to the observed Q- and U-light curves. The relationship between the scintillating components and the VLBI components are also discussed. It is shown that Doppler beaming of the 20 cm scintillating components with Lorentz factor greater than or similar to 7 may be needed to reduce their intrinsic brightness temperatures significantly below the inverse-Compton limit.