The Planck-ATCA Co-eval Observations project: the bright sample

The Planck Australia Telescope Compact Array (Planck-ATCA) Co-eval Observations (PACO) have provided flux density measurements of well-defined samples of Australia Telescope 20 GHz (AT20G) radio sources at frequencies below and overlapping with Planck frequency bands, almost simultaneously with Planck observations. We have observed with the ATCA a total of 482 sources in the frequency range between 4.5 and 40 GHz in the period between 2009 July and 2010 August. Several sources were observed more than once. In this paper we present the aims of the project, the selection criteria, and the observation and data reduction procedures. We also discuss the data in total intensity for a complete sample of 189 sources with S-20 GHz > 500 mJy, Galactic latitude |b| > 5 degrees and declination delta < -30 degrees, and some statistical analysis of the spectral behaviour and variability of this sample, referred to as the 'bright PACO sample'. Finally, we discuss how these data could be used to transfer absolute calibrations to ground-based telescopes using the cosmic microwave background dipole calibrated flux densities measured by the Planck satellite, and we provide some test fluxes on bright calibrators.