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

Abstract

Ground-based optical polarimetry and photometry of the BL Lac object PKS 2155-304 conducted just before and during the IUE monitoring program from November 1 to 19, 1991 are presented. The optical observations reveal strong daily variability in both flux and linear polarization. Broadband 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 two weeks of November. The degree of polarization varied between 2 and 8 percent, and fluctuations of 2-3 percent were observed in a day. Daily variations in the position angle of the polarization were observed to be as high as 25 percent, though the position angle was observed to lie between 105 and 140 deg throughout the monitoring program. Some of the implications that these optical data have on models of the source of the continuum emission in PKS 2155-304 are discussed.