POLAMI: Polarimetric Monitoring of AGN at Millimetre Wavelengths - I. The programme, calibration and calibrator data products

We describe the POLAMI (Polarimetric Monitoring of AGN at Millimetre Wavelengths) programme for the monitoring of all four Stokes parameters of a sample of bright radio-loud active galactic nuclei with the IRAM 30-m telescope at 3.5 and 1.3 mm. The programme started in 2006 October and accumulated, until 2014 August, 2300 observations at 3.5 mm, achieving a median time sampling interval of 22 d for the sample of 37 sources. This first paper explains the source selection, mostly blazars, the observing strategy and data calibration and gives the details of the instrumental polarization corrections. The sensitivity (1σ) reached at 3.5 mm is 0.5 per cent (linear polarization degree), 4.7° (polarization angle), and 0.23 per cent (circular polarization), while the corresponding values at 1.3 mm are 1.7 per cent, 9.9° and 0.72 per cent, respectively. The data quality is demonstrated by the time sequences of our calibrators Mars and Uranus. For the quasar 3C 286, widely used as a linear polarization calibrator, we give improved estimates of its linear polarization, and show for the first time occasional detections of its weak circular polarization, which suggests a small level of variability of the source at millimeter wavelengths.