Statistics of the fractional polarization of compact radio sources in Planck maps

In this work, we apply the stacking technique to estimate the average fractional polarization from 30 to 353 GHz of a primary sample of 1560 compact sources - essentially all radio sources - detected in the 30 GHz Planck all-sky map and listed in the second version of the Planck Catalogue of Compact Sources (PCCS2). We divide our primary sample in two subsamples according to whether the sources lay (679 sources) or not (881 sources) inside the sky region defined by the Planck Galactic mask (f(sky) similar to 60 per cent) and the area around the Magellanic Clouds. We find that the average fractional polarization of compact sources is approximately constant (with frequency) in both samples (with a weighted mean over all the channels of 3.08 per cent outside and 3.54 per cent inside the Planck mask). In the sky region outside the adopted mask, we also estimate the mu and sigma parameters for the lognormal distribution of the fractional polarization, finding a weighted mean value over all the Planck frequency range of 1.0 for sigma and 0.7 for mu (that would imply a weighted mean value for the median fractional polarization of 1.9 per cent).