Intensive optical monitoring of the BL lacertae object PKS 2155-304

We present ground-based optical polarimetry and photometry of the BL Lac object PKS 2155-304. These observations were taken just before and during the IUE monitoring program which lasted from 1991 November 1 to November 29 (UT). The optical observations reveal strong daily variability in both flux and linear polarization. Broad-band UBVRI photometry shows that the slope of the optical continuum remained relatively constant even though the object brightened by nearly a magnitude. PKS 2155-304 was near its historical photometric maximum during the last 2 weeks of November. The degree of polarization varied between 2% and 8% and fluctuations of 2%-3% were observed in a day. Daily variations in the position angle of the polarization were observed to be as high as 25°, though the position angle was observed to lie between 105° and 140° throughout the monitoring program. PKS 2155-304 exhibited mild wavelength dependence in its optical polarization with the object being slightly more polarized in the blue than the red. There is little evidence for substantial variability of this wavelength dependence. Wavelength dependence in the polarization position angle is rare and very weak when present. No consistent connection between brightness and polarization is found in our observations of this BL Lac object. The polarized optical emission varies more rapidly than the total optical flux, and the amplitude and characteristic time scale of the polarization variations are similar to those reported for the soft X-ray emission from this object. We discuss some of the implications that these optical data have on models of the source of the continuum emission in PKS 2155-304.