Refractive Focusing by Interstellar Clouds and the Rapid Polarization Angle Swing in QSO 1150+812

A very rapid polarization position angle swing of ∼ 180 ◦ (with a time scale of ∼6 hours) observed at 2cm in QSO1150+812 (z = 1.25) was reported by Kochenov & Gabuzda. This very rare event is difficult to explain. We found a pos- sible 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 spe- cific 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 po- larized components are estimated to have brightness temperatures of ∼ 6 × 10 12 K. Thus a bulk relativistic motion with a Lorentz factor less than 10 is required to meet the inverse-Compton limit.