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Abstract:
The extragalactic background light (EBL) is the second most intense diffuse component in the Universe after the CMB. Though direct measurements of the EBL density remain difficult, stringent upper-limits have been set with distant, very high energy (VHE, E > 100 GeV) blazars. These constrains have been, until now, based on limiting assumptions about the shape and hardness of the intrinsic spectra of the sources, either derived from theoretical modelling or involving proxies such as the Fermi-LAT spectrum at lower energies. In this study, the extrinsic EBL absorption and the intrinsic spectra of the sources are jointly fitted with a maximum likelihood approach. Intrinsic curvature is accounted for, under the minimal assumption of smoothness. This innovative method, applied to the high-quality spectra of the brightest blazars observed by H.E.S.S., yields the first firm detection of an EBL imprint at VHE.
