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The Extragalactic Background Light (EBL) is the integrated light from all thestars that have ever formed, and spans the IR-UV range. The interaction ofvery-high-energy (VHE: E>100 GeV) gamma-rays, emitted by sources located atcosmological distances, with the intervening EBL results in electron-positronpair production that leads to energy-dependent attenuation of the observed VHEflux. This introduces a fundamental ambiguity in the interpretation of measuredVHE gamma-ray spectra: neither the intrinsic spectrum, nor the EBL, areseparately known -- only their combination is. In this paper we propose amethod to measure the EBL photon number density. It relies on usingsimultaneous observations of BL Lac objects in the optical, X-ray, high-energy(HE: E>100 MeV) gamma-ray (from the Fermi telescope), and VHE gamma-ray (fromCherenkov telescopes) bands. For each source, the method involves best-fittingthe spectral energy distribution (SED) from optical through HE gamma-rays (thelatter being largely unaffected by EBL attenuation as long as z<1) with aSynchrotron Self-Compton (SSC) model. We extrapolate such best-fitting modelsinto the VHE regime, and assume they represent the BL Lacs' intrinsic emission.Contrasting measured versus intrinsic emission leads to a determination of thephoton-photon opacity to VHE photons. Using, for each given source, differentstates of emission will only improve the accuracy of the proposed method. Wedemonstrate this method using recent simultaneous multi-frequency observationsof the high-frequency-peaked BL Lac object PKS 2155-304 and discuss how similarobservations can more accurately probe the EBL.

the astrophysical journal. letters

High-frequency-peaked BL Lacertae objects as spectral candles to measure  the Extragalactic Background Light in the Fermi and air Cherenkov telescopes  era