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 Ñickering of 0.2È0.3 mag on timescales of 1È2 days, superposed on longer timescale variations. Rapid variations of D0.01 mag minute~1, if real, are the fastest seen to date for any BL Lac object. Small (0.2È0.3 mag) increases in the V and R bands occur simultaneously with a Ñare seen at higher energies. All optical wave bands (UBV RI) 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, and the highest optical polariza-P U /P I \ 1.31) tion (U \ 14.3%) ever observed for this source. The polarization variations trace the Ñares seen in the UV Ñux. For the fastest variability timescale observed, we estimate a central black hole mass of [1.5 ] 109(d/10) consistent with UV and X-ray constraints and smaller than previously calculated for M _ , this object.
A B S T R A C TNear-infrared photometry was performed on 56 southern 6.7-GHz methanol maser sources. A simple spherically symmetric model of the radiative transfer through a dust shell was developed and used to study the conditions in the dust cloud in which the masers are produced. The parameters investigated were the size of the cloud, the spectral type of the embedded star, the optical depth of the dust cloud and the dust density distribution. It was found that the infrared colours of the models have a complex dependence on the parameters and that no unique combination of parameter values explains the spectral energy distribution of any particular source. The model effectively reproduces the far-infrared (IRAS) colours but cannot simultaneously explain the near-infrared colours for any of the observed sources.
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