The Planck-ATCA Co-eval Observations (PACO) project: the spectrally-selected sample

TL;DR

This study analyzes multi-frequency radio observations of a complete sample of bright sources, revealing spectral behaviors, variability patterns, and the rarity of high-frequency peaking sources, contributing to understanding radio source evolution.

Contribution

It provides the first detailed spectral and variability analysis of a spectrally-selected complete sample of radio sources, identifying the rarity of HFPs and characterizing blazar spectral evolution.

Findings

HFPs are very rare (<0.5%) among bright sources.

Most blazar spectra are smooth and well described by a double power-law.

Peak frequency of sources decreases over time at an average rate of -3 GHz/yr.

Abstract

The Planck-ATCA Co-eval Observations (PACO) have provided multi-frequency (5-40 GHz) flux density measurements of complete samples of Australia Telescope 20 GHz (AT20G) radio sources at frequencies below and overlapping with Planck frequency bands, almost simultaneously with Planck observations. In this work we analyse the data in total intensity for the spectrally-selected PACO sample, a complete sample of 69 sources brighter than 200 mJy at 20 GHz selected from the AT20G survey catalogue to be inverted or upturning between 5 and 20 GHz. We study the spectral behaviour and variability of the sample. We use the variability between AT20G (2004-2007) and PACO (2009-2010) epochs to discriminate between candidate High Frequency Peakers (HFPs) and candidate blazars. The HFPs picked up by our selection criteria have spectral peaks > 10 GHz in the observer frame and turn out to be rare (<0.5% of the S >200 mJy sources), consistent with the short duration of this phase implied by the `youth' scenario. Most (~ 89 %) of blazar candidates have remarkably smooth spectra, well described by a double power-law, suggesting that the emission in the PACO frequency range is dominated by a single emitting region. Sources with peaked PACO spectra show a decrease of the peak frequency with time at a mean rate of -3 +- 2 GHz/yr on an average timescale of \tau = 2.1 +- 0.5 yr. The 5-20 GHz spectral indices show a systematic decrease from AT20G to PACO observations. At higher frequencies spectral indices steepen: the median \alpha 30-40 GHz is steeper than the median \alpha 5-20 GHz by \delta\alpha = 0.6. Taking further into account the WISE data we find that the SEDs, \nu S(\nu), of most of our blazars peak at \nu_p <10^5 GHz; the median peak wavelength is \lambda_p ~93 \mu m. Only 6 have \nu_p>10^5 GHz.