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We propose a method for setting upper limits to the extragalactic backgroundlight (EBL). Our method uses simultaneous {\em Fermi}-LAT and ground-based TeVobservations of blazars and is based on the assumption that the intrinsicspectral energy distribution (SED) of TeV blazars lies below the extrapolationof the {\em Fermi}-LAT SED from GeV to TeV energies. By extrapolating the {\emFermi}-LAT spectrum, which for TeV blazars is practically unattenuated byphoton-photon pair production with EBL photons, a firm upper limit on theintrinsic SED at TeV energies is provided. The ratio of the extrapolatedspectrum to the observed TeV spectrum provides upper limits to the opticaldepth for the propagation of the TeV photons due to pair production on the EBL,which in turn sets firm upper limits to EBL models. We demonstrate our methodusing simultaneous observations from {\em Fermi}-LAT and ground-based TeVtelescopes of the blazars \object{PKS 2155-304} and \object{1ES 1218+304}, andshow that high EBL density models are disfavored. We also discuss how ourmethod can be optimized and how {\em Fermi} and X-ray monitoring observationsof TeV blazars can guide future TeV campaigns, leading to potentially muchstronger constraints on EBL models.

A method for setting upper limits to the extragalactic background light with Fermi-LAT and TeV observations of blazars