The 2009 Fermi Symposium

Blazars as Beamlights to Probe the EBL

Massimo Persic, INAF/INFN, Trieste, Italy

Abstract:

The Extragalactic Background Light (EBL) is the integrated light from all stars that have ever formed, and spans the IR-UV range. The interaction of very-high-energy (VHE: E > 100 GeV) gamma-rays, emitted by AGNs at cosmological distances, with the intervening EBL results in electron-positron pair production that leads to an energy-dependent attenuation of the observed VHE flux. This introduces a fundamental ambiguity in the interpretation of the measured VHE blazar spectra: neither the intrinsic spectra, nor the EBL, are separately known -- only their combination is. Here we introduce a method to measure the EBL photon number density. It relies on using simultaneous observations of blazars in the optical, X-ray, high-energy (HE: E>100 MeV) gamma-ray (from the Fermi telescope), and VHE gamma-ray (from Cherenkov telescopes) bands. For each source, we best-fit the Spectral Energy Distribution (SED) from optical through HE gamma-rays (the latter being unaffetcted by EBL attenuation) with a Synchro Self-Compton (SSC) model. We extrapolate such best-fitting models into the VHE regime, and assume they represents the blazars' intrinsic VHE emission. Contrasting measured vs intrinsic emissions leads to a determination the gamma-gamma opacity to VHE photons -- hence, upon assuming a specific cosmology, we derive the EBL photons' number density. Using, for each given source, different states of emission will only improve the accuracy of the proposed method.