Refractive Focusing of Interstellar Clouds and Intraday Polarization Angle Swings

Abstract

Intraday variations of compact extragalactic radio sources in flux density and polarization are generally interpreted in terms of refractive scintillation from the continuous interstellar medium of our Galaxy. However, continuous polarization angle swings of ~180° (for example, the one observed in the QSO 0917+624) could not be interpreted in this way. Qian et al. have shown that the polarization angle swing observed in the QSO 1150+812 can be explained in terms of focusing–defocusing effect by an interstellar cloud, which occults two closely-placed polarized components. Here we further show that the polarization angle swing event observed in the QSO 0917+624 can also be explained in this way. We also found evidence for the cloud eclipsing a non-polarized (core) component during a short period outside the swing. A particular (and specific) plasma-lens model is proposed to model-fit the polarization swing event of 0917+624. Some physical parameters related to the plasma-lens and the source components are estimated. The brightness temperatures of the two lensed components are estimated to be ~1.6×1013 K. Thus bulk relativistic motion with a Lorentz factor less than ~20 may be sufficient to avoid the inverse-Compton catastrophe.