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The rapid variability of blazars is an important clue to the origin of their unusual continuum emission. Continuous UV/EUV/X-ray monitoring of the BL Lac object PKS~2155-304 over a period of 2-10 days in May 1994 has revealed unexpected and exciting results. The ASCA light curve shows a fully resolved, fast, strong X-ray flare, possibly associated with smaller flares seen with IUE and EUVE. The X-rays lead the UV photons by at least one day. The wavelength dependence of the flare amplitude and the observed lag are in sharp contrast to the results from earlier multiwavelength monitoring of the same object, which showed optical, UV, and X-ray light curves varying with the same amplitude and with a short lag. The two kinds of variability must have different physical explanations. The current data are qualitatively consistent with a synchrotron flare from a propagating shock in a relativistic jet, while it may be possible to explain the achromatic variations seen earlier by gravitational microlensing.
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