Anti-correlated Optical Flux and Polarization Variability in BL Lac
  We present the results of photometric (V band) and polarimetric observations
of the blazar BL Lac during 2008--2010 using TRISPEC attached to the KANATA
1.5-m telescope in Japan. The data reveal a great deal of variability ranging
from days to months with detection of strong variations in fractional
polarization. The V band flux strongly anti-correlates with the degree of
polarization during the first of two observing seasons but not during the
second. The direction of the electric vector, however, remained roughly
constant during all our observations. These results are consistent with a model
with at least two emission regions being present, with the more variable
component having a polarization direction nearly perpendicular to that of the
relatively quiescent region so that a rising flux can produce a decline in
degree of polarization. We also computed models involving helical jet
structures and single transverse shocks in jets and show that they might also
be able to agree with the anti-correlations between flux and fractional
polarization.
