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We report on the first simultaneous observations that cover the optical,X-ray, and high energy gamma-ray bands of the BL Lac object PKS 2155-304. Thegamma-ray bands were observed for 11 days, between 25 August and 6 September2008, jointly with the Fermi Gamma-ray Space Telescope and the H.E.S.S.atmospheric Cherenkov array, providing the first simultaneous MeV-TeV spectralenergy distribution with the new generation of gamma-ray telescopes. The ATOMtelescope and the RXTE and Swift observatories provided optical and X-raycoverage of the low-energy component over the same time period. The object wasclose to the lowest archival X-ray and Very High Energy state, whereas theoptical flux was much higher. The light curves show relatively little (~30%$)variability overall when compared to past flaring episodes, but we find a clearoptical/VHE correlation and evidence for a correlation of the X-rays with thehigh energy spectral index. Contrary to previous observations in the flaringstate, we do not find any correlation between the X-ray and VHE components.Although synchrotron self-Compton models are often invoked to explain the SEDsof BL Lac objects, the most common versions of these models are at odds withthe correlated variability we find in the various bands for PKS 2155-304.

Simultaneous observations of PKS 2155-304 with H.E.S.S., Fermi, RXTE and ATOM: spectral energy distributions and variability in a low state