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