Multiwavelength Monitoring of the BL Lacertae Object PKS 2155−304 in 1994 May. I. The Ground‐based Campaign

Optical, near-infrared, and radio observations of the BL Lac object PKS 2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray monitoring campaign in 1994 May. Further optical observations were gathered throughout most of 1994. The radio, millimeter, and near-infrared data show no strong correlations with the higher energies. The optical light curves exhibit flickering of 0.2-0.3 mag on timescales of 1-2 days, superposed on longer timescale variations. Rapid variations of ~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 bands occur simultaneously with a flare seen at higher energies. All optical wave bands ( UBVRI ) track each other well over the period of observation, with no detectable delay. For most of the period the average colors remain relatively constant, although there is a tendency for the colors (in particular, B - V ) to vary more when the source fades. In polarized light, PKS 2155-304 showed strong color dependence (polarization increases toward the blue, P U / P I = 1.31) and the highest optical polarization ( U = 14.3%) ever observed for this source. The polarization variations trace the flares seen in the UV flux. For the fastest variability timescale observed, we estimate a central black hole mass of ≲1.5 × 10 9 (δ/10) M ☉ , consistent with UV and X-ray constraints and smaller than previously calculated for this object.