A 3.5mm Polarimetric Survey of Radio-loud Active Galactic Nuclei

TL;DR

This study presents the first large-scale 3.5 mm polarimetric survey of radio-loud AGN, revealing polarization properties, jet luminosity relations, and implications for cosmic microwave background measurements.

Contribution

It provides new polarimetric data at 3.5 mm for over 100 AGN, highlighting polarization characteristics and jet properties not previously characterized at this wavelength.

Findings

76% of sources show linear polarization above 3 sigma

Linear polarization at 86 GHz is about twice that at 15 GHz

Jet luminosity inversely correlates with linear polarization

Abstract

We present the results from the first large (>100 source) 3.5 mm polarimetric survey of radio loud active galactic nuclei (AGN). The I, Q, U, and V Stokes parameter observations were performed with the XPOL polarimeter at the IRAM 30m Telescope. Our sample consists of 145 flat-radio-spectrum AGN with declination >-30 deg. (J2000.0) and flux density >~1Jy at 86GHz, as measured at the IRAM 30 m Telescope from 1978 to 1994. We detected linear and circular polarization (above 3 sigma levels of ~1.6%, and ~0.3%) for 76%, and 6% of the sample, respectively. We find a clear excess in degree of linear polarization detected at 86 GHz with regard to that at 15 GHz by a factor of ~2. Over our entire source sample, the luminosity of the jets is inversely proportional to the degree of linear polarization. Consistent with previous findings claiming larger Doppler factors for brighter gamma-ray blazars, quasars listed in our sample, and in the Fermi-LBAS catalog, show larger luminosities than non-LBAS ones, but our data do not allow us to confirm the same for BL Lac objects. We do not find a clear relation between the linear polarization angle and the jet structural position angle for any source class in our sample. We interpret this as the consequence of a markedly non-axisymmetric character of the 3 mm emitting region in the jets. We find that intrinsic circular polarization is the most likely mechanism for generation of the circular polarization detected in our observations. Our new data can be used to estimate the 3 mm AGN contribution to measurements of the linear polarization of the cosmic microwave background, such as those performed by the Planck satellite.