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_v/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 × 10⁹(δ/10) M_⊙, consistent with UV and X-ray constraints and smaller than previously calculated for this object.