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

TL;DR

This study presents the first simultaneous multi-wavelength observations of PKS 2155-304 across optical, X-ray, and gamma-ray bands during a low activity state, revealing new insights into its spectral energy distribution and variability patterns.

Contribution

It provides the first simultaneous MeV-TeV spectral energy distribution of PKS 2155-304 and analyzes its variability, challenging existing synchrotron self-Compton models.

Findings

Optical/VHE correlation observed

X-ray variability correlates with spectral index

No correlation between X-ray and VHE during low state

Abstract

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. The gamma-ray bands were observed for 11 days, between 25 August and 6 September 2008, jointly with the Fermi Gamma-ray Space Telescope and the H.E.S.S. atmospheric Cherenkov array, providing the first simultaneous MeV-TeV spectral energy distribution with the new generation of gamma-ray telescopes. The ATOM telescope 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 X-ray and Very High Energy state, whereas the optical flux was much higher. The light curves show relatively little (~30%$) variability overall when compared to past flaring episodes, but we find a clear optical/VHE correlation and evidence for a correlation of the X-rays with the high energy spectral index. Contrary to previous observations in the flaring state, we do not find any correlation between the X-ray and VHE components. Although synchrotron self-Compton models are often invoked to explain the SEDs of BL Lac objects, the most common versions of these models are at odds with the correlated variability we find in the various bands for PKS 2155-304.