Vol. 550
In section 4. Extragalactic astronomy

Measurement of the extragalactic background light imprint on the spectra of the brightest blazars observed with H.E.S.S.

Determining the intrinsic properties of the extragalactic background light (EBL), which is the cumulative radiative product of cosmic galactic evolution, is one of the basic problems of cosmology. The present paper presents a novel approach to its recovery using H.E.S.S. Atmospheric Cherenkov Telescope observations of the very high-energy, TeV gamma rays from a well studied sample of blazars. The spectra of the individual sources are modified by passage through the EBL through photon-photon interactions due to the cosmic-interaction mean free path for the source photons, equivalent to an optical depth with a known energy-dependent cross section that depends on the number density of the EBL photons. The authors apply maximum likelihood analysis to simultaneously recover the intrinsic source spectrum and energy-dependent optical depth for a range of assumed parametric forms for the intrinsic source spectrum. The resulting EBL spectrum peaks at around 1.4 microns with a minimum at around 10 microns and with a significance of 8.8 sigma for the detected EBL optical depth.