MULTIWAVELENGTH MONITORING OF THE BL LACERTAE OBJECT PKS-2155-304 IN 1994 MAY .1. THE GROUND-BASED CAMPAIGN

Optical, near-infrared, and radio observations of the BL Lac object PK S 2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray monitoring campaign in 1994 May. Further optical observations were ga thered throughout most of 1994. The radio, millimeter, and near-infrar ed data show no strong correlations with the higher energies. The opti cal light curves exhibit flickering of 0.2-0.3 mag on timescales of 1- 2 days, superposed on longer timescale variations. sapid variations of similar to 0.01 mag minute(-1), if real, are the fastest seen to date for any BL Lac object. Small (0.2-0.3 mag) increases in the V and R b ands occur simultaneously with a flare seen at higher energies. All op tical wave bands (UBVRI) track each other well over the period of obse rvation, with no detectable delay. For most of the period the average colors remain relatively constant, although there is a tendency for th e colors (in particular, B-V) to vary more when the source fades. In p olarized light, PKS 2155-304 showed strong color dependence (polarizat ion increases toward the blue, P-U/P-I = 1.31) and the highest optical polarization (U = 14.3%) ever observed for this source. The polarizat ion variations trace the flares seen in the UV flux. For the fastest v ariability timescale observed, we estimate a central black hole mass o f less than or similar to 1.5 x 10(9)(delta/10) M-., consistent with U V and X-ray constraints and smaller than previously calculated for thi s object.