Interstellar Scintillation of the Polarized Flux Density in Quasar PKS 0405–385

The remarkable rapid variations in radio flux density and polarization of the quasar PKS 0405-385 observed in 1996 are subject to a correlation analysis, from which characteristic timescales and amplitudes are derived. The variations are interpreted as interstellar scintillations (ISSs). The centimeter wavelength observations are in the weak scintillation regime for which models for the various auto- and cross-correlations of the Stokes parameters are derived and fitted to the observations. These are well modeled by ISS of a 30 × 22 μas source, with about 180° rotation of the polarization angle along its long dimension. This success in explaining the remarkable intraday variability (IDVs) in polarization confirms that ISS gives rise to the IDV in this quasar. However, the fit requires the scintillations to be occurring much closer to the Earth than expected according to the standard model for the ionized interstellar medium (IISM). Scattering at distances in the range 3-30 pc is required to explain the observations. For our preferred distance of 25 pc the associated source model has a peak brightness temperature near 2 × 1013 K, which is about 25 times smaller than previously derived for this source. This reduces the implied Doppler factor in the relativistic jet, presumed responsible to ~75, which is still substantially higher than centimeter wavelength VLBI estimates for the Doppler factors in active galactic nuclei (AGNs).