# Statistics of the fractional polarisation of compact radio sources in Planck maps

## TL;DR

This study uses stacking techniques on Planck data to measure the average fractional polarisation of compact radio sources across multiple frequencies, revealing a roughly constant polarisation level around 3%.

## Contribution

It provides the first comprehensive measurement of the fractional polarisation of compact radio sources from 30 to 353 GHz using stacking on Planck data, including distribution parameters.

## Key findings

- Average fractional polarisation is approximately 3% across frequencies.
- Fractional polarisation distribution follows a log-normal with specific parameters.
- Polarisation levels are similar inside and outside the Galactic mask.

## Abstract

In this work we apply the stacking technique to estimate the average fractional polarisation 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 (fsky ~ 60 per cent) and the area around the Magellanic Clouds. We find that the average fractional polarisation 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 log-normal distribution of the fractional polarisation, 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 polarisation of 1.9 per cent).