Astronomy and Astrophysics, volume 566A, 59-59 (2014/6-1)
A simultaneous 3.5 and 1.3 mm polarimetric survey of active galactic nuclei in the northern sky.
AGUDO I., THUM C., GOMEZ J.L. and WIESEMEYER H.
Abstract (from CDS):
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. We present the first simultaneous dual frequency 86GHz and 229GHz polarimetric survey of all four Stokes parameters for a large sample of 211 radio-loud active galactic nuclei, designed to be flux limited at 1Jy at 86GHz. Most of the observations were made in mid-August 2010 using the XPOL polarimeter on the IRAM 30m millimetric radio telescope. Linear polarization detections above a 3σ median level of ∼1.0% are reported for 183 sources at 86GHz and for 23 sources at 229GHz, where the median 3σ level is ∼6.0%. We show a clear excess of the linear polarization degree that is detected at 229GHz with regard to that at 86GHz 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 229GHz 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 86GHz 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.
galaxies: active - galaxies: jets - quasars: general - BL Lacertae objects: general - polarization - surveys
VizieR on-line data:
<Available at CDS (J/A+A/566/A59): table1.dat table2.dat>