A simultaneous 3.5 and 1.3 mm polarimetric survey of active galactic nuclei in the northern sky^⋆

¹ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
² Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
³ Current Address: Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
e-mail: agudo@jive.nl
⁴ Instituto de Radio Astronomía Millimétrica, Avenida Divina Pastora, 7, Local 20, 18012 Granada, Spain
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel, 69, 53121 Bonn, Germany

Received: 2 January 2014
Accepted: 11 April 2014

Abstract

Context. Short millimeter observations of radio-loud active galactic nuclei (AGN) offer an excellent opportunity to study the physics of their synchrotron-emitting relativistic jets from where the bulk of radio and millimeter emission is radiated. On one hand, AGN jets and their emission cores are significantly less affected by Faraday rotation and depolarization than at longer wavelengths. On the other hand, the millimeter emission of AGN is dominated by the compact innermost regions in the jets, where the jet cannot be seen at longer wavelengths due to synchrotron opacity.

Aims. We present the first simultaneous dual frequency 86 GHz and 229 GHz polarimetric survey of all four Stokes parameters for a large sample of 211 radio-loud active galactic nuclei, designed to be flux limited at 1 Jy at 86 GHz.

Methods. Most of the observations were made in mid-August 2010 using the XPOL polarimeter on the IRAM 30 m millimetric radio telescope.

Results. Linear polarization detections above a 3σ median level of ~ 1.0% are reported for 183 sources at 86 GHz and for 23 sources at 229 GHz, where the median 3σ level is ~ 6.0%. We show a clear excess of the linear polarization degree that is detected at 229 GHz with regard to that at 86 GHz by a factor of ~ 1.6. This implies a progressively better ordered magnetic field for blazar jet regions that are located progressively upstream in the jet. We show that the linear polarization angle at 86 and 229 GHz and the jet structural position angle for both quasars and BL Lacs do not show a clear preference to align in either parallel or perpendicular directions. Our variability study with regard to the 86 GHz data from our previous survey points out a large degree of variation. In particular, we report total flux and linear polarization changes in time scales of years by median factors of ~ 1.5 in total flux and ~ 1.7 in linear polarization degree (with maximum variations by factors up to 6.3 and ~ 5, respectively). Moreover, 86% of sources show linear polarization angles evenly distributed with regard to our previous measurements.