Upper Limits on the Extragalactic Background Light from the Very High Energy Gamma‐Ray Spectra of Blazars

The direct measurement of the extragalactic background light (EBL) isdifficult at optical to infrared wavelengths because of the strong foregroundradiation originating in the Solar System. Very high energy (VHE, E$>$100 GeV)gamma rays interact with EBL photons of these wavelengths through pairproduction. In this work, the available VHE spectra from six blazars are usedto place upper limits on the EBL. These blazars have been detected over a rangeof redshifts and a steepening of the spectral index is observed with increasingsource distance. This can be interpreted as absorption by the EBL. In general,knowledge of the intrinsic source spectrum is necessary to determine thedensity of the intervening EBL. Motivated by the observed spectral steepeningwith redshift, upper limits on the EBL are derived by assuming that theintrinsic spectra of the six blazars are $\propto E^{-1.8}$. Upper limits arethen placed on the EBL flux at discrete energies without assuming a specificspectral shape for the EBL. This is an advantage over other methods since theEBL spectrum is uncertain.Comment: 33 pages, 14 figures, accepted by Ap