Abstract:
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
anticorrelates 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 of 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 anticorrelations between flux and
fractional polarization.
