Abstract A very rapid polarization position angle swing of ~180° (with a time scale of ~6 hours) observed at 2 cm in QSO 1150+812 (z = 1.25) was reported by Kochenov & Gabuzda. This very rare event is difficult to explain. We found a possible interpretation in the framework of a source model consisting of three polarized components, in which two compact polarized components are nearly simultaneously occulted by an interstellar cloud, with consequent focusing-defocusing effects. A specific plasma-lens model is proposed which can reasonably fit the polarized flux density curve with results derived for the two lensed components. Some physical parameters of the plasma-lens and the source components are estimated. The two compact polarized components are estimated to have brightness temperatures of ~6×1012 K. Thus a bulk relativistic motion with a Lorentz factor less than 10 is required to meet the inverse–Compton limit.
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