Polarization Limits in Compact Symmetric Objects

Abstract

We present results of multifrequency polarimetric VLBA observations of 34 compact radio sources. The observations are part of a large survey undertaken to identify CSOs Observed in the Northern Sky (COINS). Based on VLBI continuum surveys of ~2000 compact radio sources, we have defined a sample of 52 CSOs and CSO candidates. Positive identification of CSOs is contingent upon acquiring multi-frequency observations in order to correctly identify the core of the source, which is expected to have a strongly inverted spectrum. We also expect CSOs to exhibit very little polarized flux. Despite the fact that synchrotron emission is intrinsically highly polarized, less than 0.5% fractional polarization is seen in low resolution studies of CSOs at frequencies up to 5 GHz. One possible explanation for the low observed linear polarization from CSOs is that their radiation is depolarized as it passes through a magnetized plasma associated with a circumnuclear torus. This interpretation is consistent with the unified scheme of AGN, and also with the recent detections of free-free and HI absorption in CSOs. Here we present limits on the polarized flux density at 8.4 GHz and ~1 mas resolution for 21 CSOs and candidates in the COINS sample.