The Planck Australia Telescope Compact Array ( 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 ( SS ) PACO sample , a complete sample of 69 sources brighter than S _ { 20 GHz } = 200 mJy 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 _ { 20 GHz } \geq 200 mJy sources ) , consistent with the short duration of this phase implied by the ‘ youth ’ scenario .
Most ( \simeq 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 \pm 2 \hbox { GHz } \hbox { yr } ^ { -1 } on an average timescale of \langle \tau \rangle = 2.1 \pm 0.5 yr ( median : \tau _ { median } = 1.3 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 } is steeper than the median \alpha _ { 5 } ^ { 20 } by \delta \alpha = 0.6 .
Taking further into account the Wide-field Infrared Survey Explorer ( WISE ) data we find that the Spectral Energy Distributions ( SEDs ) , \nu S ( \nu ) , of most of our blazars peak at \nu _ { p } ^ { SED } < 10 ^ { 5 } GHz ; the median peak wavelength is \lambda _ { p } ^ { SED } \simeq 93 \mu m. Only 6 have \nu _ { p } ^ { SED } > 10 ^ { 5 } GHz .