MOJAVE: Monitoring of Jets in AGN with VLBA Experiments - I. First-Epoch 15 GHz Linear Polarization Images

Abstract

We present first-epoch, milliarcsecond-scale linear polarization images at 15 GHz of 133 jets associated with AGN in the MOJAVE survey (Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments). The sample consists of all known AGN with galactic latitude |b| > 2.5 deg., J2000 declination > -20 deg., and 15 GHz VLBA flux density exceeding 1.5 Jy (2 Jy for sources with declination < 0) at any epoch during the period 1994-2003. Because of strong selection effects, the sample primarily consists of blazars with parsec-scale morphologies consisting of a bright core component at the extreme end of a one-sided jet. At least one third of the compact cores are completely unresolved on the longest VLBA baselines, indicating brightness temperatures above 10^{11} K. The unresolved cores tend to have electric vectors that are aligned with the inner jet direction, suggesting the presence of a stationary transverse shock near the base of the jet. Many of the extended jet regions display exceedingly high fractional polarizations (> 50%) and electric vectors aligned with the jet ridge line, consistent with optically thin emission from transverse shocks. Both weak- and strong-lined blazars show a general increase in fractional polarization with distance down the jet, but BL Lac jets are generally more polarized and have electric vectors preferentially aligned with the local jet direction. We show that these differences are intrinsic to the jets, and not due to sample bias. Distinct features in the jets of gamma-ray loud (EGRET) blazars are typically twice as luminous as those in non-EGRET blazars, and are more polarized. These differences can be adequately explained if EGRET blazars have slightly higher Doppler boosting factors than those not yet detected in gamma-rays.