Optical polarization of high-energy BL Lac objects
  We investigate the optical polarization properties of high-energy BL Lac
objects using data from the RoboPol blazar monitoring program and the Nordic
Optical Telescope. We wish to understand if there are differences in the BL Lac
objects that are detected with the current-generation TeV instruments compared
to those that have not yet been detected. The mean polarization fraction of the
TeV-detected BL Lacs is 5% while the non-TeV sources show a higher mean
polarization fraction of 7%. This difference in polarization fraction
disappears when the dilution by the unpolarized light of the host galaxy is
accounted for. The TeV sources show somewhat lower fractional polarization
variability amplitudes than the non-TeV sources. Also the fraction of sources
with a smaller spread in the Q/I - U/I -plane and a clumped distribution of
points away from the origin, possibly indicating a preferred polarization
angle, is larger in the TeV than in the non-TeV sources. These differences
between TeV and non-TeV samples seems to arise from differences between
intermediate and high spectral peaking sources instead of the TeV detection.
When the EVPA variations are studied, the rate of EVPA change is similar in
both samples. We detect significant EVPA rotations in both TeV and non-TeV
sources, showing that rotations can occur in high spectral peaking BL Lac
objects when the monitoring cadence is dense enough. Our simulations show that
we cannot exclude a random walk origin for these rotations. These results
indicate that there are no intrinsic differences in the polarization properties
of the TeV-detected and non-TeV-detected high-energy BL Lac objects. This
suggests that the polarization properties are not directly related to the
TeV-detection, but instead the TeV loudness is connected to the general flaring
activity, redshift, and the synchrotron peak location. (Abridged)
