3C 286: a bright, compact, stable, and highly polarized calibrator for millimeter-wavelength observations

¹ Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
e-mail: iagudo@iaa.es
² Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
³ Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 St. Martin d’Hères, France
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Instituto de Radio Astronomía Milimétrica, Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
⁶ School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

Received: 10 January 2012
Accepted: 29 February 2012

Abstract

Context. Several millimeter and submillimeter facilities with linear polarization observing capabilities have started operating during the last years. These facilities, as well as other previous millimeter telescopes and interferometers, require bright and stable linear polarization calibrators to calibrate new instruments and to monitor their instrumental polarization. The current limited number of adequate calibrators implies difficulties in the acquisition of these calibration observations.

Aims. Looking for additional linear polarization calibrators in the millimeter spectral range, we started monitoring 3C 286 in mid-2006. This source is a standard and highly stable polarization calibrator for radio observations.

Methods. Here we present the 3 mm and 1 mm monitoring observations obtained between September 2006 and January 2012 with the XPOL polarimeter on the IRAM 30 m Millimeter Telescope.

Results. Our observations show that 3C 286 is a bright source of constant total flux with 3 mm flux density S_3   mm = (0.91 ± 0.02) Jy. The 3 mm linear polarization degree (p_3   mm = [13.5 ± 0.3] %) and polarization angle (χ_3   mm = [37.3 ± 0.8] °, expressed in the equatorial coordinate system) were also constant during the time span of our observations. Although with poorer time sampling and signal-to-noise ratio, our 1 mm observations of 3C 286 are also reproduced by a constant source of 1 mm flux density (S_1   mm = [0.30 ± 0.03]  Jy), polarization fraction (p_1   mm = [14.4 ± 1.8] %), and polarization angle (χ_1   mm = [33.1 ± 5.7] °).

Conclusions. This, together with the previously known compact structure of 3C 286 – extended by  ~3.5′′ in the sky – allow us to propose 3C 286 as a new calibrator for both single-dish and interferometric polarization observations at 3 mm, and possibly at shorter wavelengths.