Multi-Frequency Polarisation Measurements of PACO Radio Sources

Active Galactic Nuclei (AGN) inner regions, poorly characterized so far at frequencies above 20 GHz, can now be studied through their emission and magnetic field properties by means of multi-frequency and multi-epoch polarimetry at centimetric and millimetric wavelengths. AGNs also constitute the main foreground for cosmic microwave background radiation on scales smaller than 30 arcmin up to 100 GHzWe present the analysis of high sensitivity polarimetric observations (< 1 mJy) for a complete sample of 106 compact extragalactic radio sources drawn from the faint (> 200 mJy at 20 GHz in total intensity) Planck-ATCA Co-eval Observations (PACO) catalogue, performed with the Australia Telescope Compact Array (ATCA) at 7 frequencies, over the 1.1-39 GHz frequency range. A currently ongoing ALMA follow-up might extend the analysis to 100 GHzPolarisation spectra of single sources cannot be simply inferred from total intensity ones, as different source components dominate the different emissions. On average, spectra steepen in both total intensity and polarisation at frequencies > 30 GHz. We characterized different behaviours in polarisation between sources peaking at cm (steep spectrum sources in our regime) or mm bands (flat and peaking spectrum sources) in total intensity: an increasing polarisation fraction with frequency emerges for steep spectrum sources and no significant trend is found for other spectral classes. Multi-epoch variability analysis (from 1.5 up to 12 yr time lags) is also presented.