Achromatic variability in the BL Lac object PKS 2155-304. A case for microlensing?

PKS 2155-304 is the only BL Lac object for which well-sampled multiwavelength light curves resolve the intraday variability at UV and X-ray wavelengths. In particular we focus on the multifrequency campaign of November 1991, which showed a rather exceptional behaviour of the source exhibiting substantial achromatic variability from optical to X-ray wavelengths. We suggest a scenario for this unique event, where the variability is due to microlensing. The relativistic motion of the source relative to the lens is taken into account. The lenses are proposed to be solar-mass stars in an intervening dwarf galaxy. The a priori probability of detecting a microlensing event, however, was only few percent, small but not negligible.