RoboPol: The optical polarization of gamma-ray--loud and
  gamma-ray--quiet blazars
  We present average R-band optopolarimetric data, as well as variability
parameters, from the first and second RoboPol observing season. We investigate
whether gamma- ray--loud and gamma-ray--quiet blazars exhibit systematic
differences in their optical polarization properties. We find that
gamma-ray--loud blazars have a systematically higher polarization fraction
(0.092) than gamma-ray--quiet blazars (0.031), with the hypothesis of the two
samples being drawn from the same distribution of polarization fractions being
rejected at the 3{\sigma} level. We have not found any evidence that this
discrepancy is related to differences in the redshift distribution, rest-frame
R-band lu- minosity density, or the source classification. The median
polarization fraction versus synchrotron-peak-frequency plot shows an envelope
implying that high synchrotron- peaked sources have a smaller range of median
polarization fractions concentrated around lower values. Our gamma-ray--quiet
sources show similar median polarization fractions although they are all low
synchrotron-peaked. We also find that the random- ness of the polarization
angle depends on the synchrotron peak frequency. For high synchrotron-peaked
sources it tends to concentrate around preferred directions while for low
synchrotron-peaked sources it is more variable and less likely to have a pre-
ferred direction. We propose a scenario which mediates efficient particle
acceleration in shocks and increases the helical B-field component immediately
downstream of the shock.
