2006ApJ...651..782Z

The optical/UV and X-ray instruments on board XMM-Newton provide an excellent opportunity to perform simultaneous observations of violently variable objects over a broad wavelength range. The UV- and X-ray-bright BL Lac object PKS 2155-304 has been observed repeatedly with XMM-Newton about twice per year. In this paper, we present a detailed analysis of the simultaneous multiwavelength variability of the source from optical to X-rays, based on the currently available XMM-Newton observations. These observations probed the intraday multiwavelength variability at optical/UV and X-ray wavelengths of the source. The UV variability amplitude is substantially smaller than the X-ray one, and the hardness ratios of the UV to X-rays correlates with the X-ray fluxes: the brighter the source, the flatter the UV/X-ray spectra. On 2000 May 30-31, the UV and X-ray light curves were weakly correlated, while the UV variations followed the X-ray ones with no detectable lags on 2000 November 19-21. On 2001 November 30, the source exhibited a major X-ray flare that was not detected in the optical. The intraday UV and X-ray variability presented here is not similar to the interday UV and X-ray variability obtained from the previous coordinated extensive multiwavelength campaigns on the source, indicating that different ``modes'' of variability might be operating in PKS 2155-304 on different timescales or from epoch to epoch.