SIMULTANEOUS OBSERVATIONS OF PKS 2155–304 WITH HESS, <i>FERMI</i> , <i>RXTE</i> , AND ATOM: SPECTRAL ENERGY DISTRIBUTIONS AND VARIABILITY IN A LOW STATE

Abstract

We report on the first simultaneous observations that cover the optical, X-ray, and high-energy gamma-ray bands&#13;\nof the BL Lac object PKS 2155−304. The gamma-ray bands were observed for 11 days, between 2008 August 25&#13;\nand 2008 September 6 (MJD 54704−54715), jointly with the Fermi Gamma-ray Space Telescope and the HESS&#13;\natmospheric Cherenkov array, providing the first simultaneousMeV–TeVspectral energy distribution (SED) with the newgeneration of γ -ray telescopes. TheATOMtelescope and the RXTE and Swift observatories provided optical and X-ray coverage of the low-energy component over the same time period. The object was close to the lowest archival&#13;\nX-ray and very high energy (VHE;&amp;gt;100 GeV) state, whereas the optical fluxwasmuch higher. The light curves show&#13;\nrelatively little (∼30%) variability overall when compared to past flaring episodes, but we find a clear optical/VHE&#13;\ncorrelation and evidence for a correlation of the X-rays with the high-energy spectral index. Contrary to previous&#13;\nobservations in the flaring state, we do not find any correlation between the X-ray and VHE components. Although&#13;\nsynchrotron self-Compton models are often invoked to explain the SEDs of BL Lac objects, the most common&#13;\nversions of these models are at odds with the correlated variability we find in the various bands for PKS 2155−304.