High-frequency-peaked BL Lacertae objects as spectral candles to measure the extragalactic background light in the fermi and air cherenkov telescopes era

摘要

The extragalactic background light (EBL) is the integrated light from all the stars that have ever formed, and spans the IR-UV range. The interaction of very high-energy (VHE: E > 100 GeV) γ-rays, emitted by sources located at cosmological distances, with the intervening EBL results in e ~- e ~+ pair production that leads to energy-dependent attenuation of the observed VHE flux. This introduces a fundamental ambiguity into the interpretation of measured VHE γ-ray spectra: neither the intrinsic spectrum nor the EBL are separately known - only their combination is. In this Letter, we propose a method to measure the EBL photon number density. It relies on using simultaneous observations of BLLac objects in the optical, X-ray, high-energy (HE: E > 100 MeV) γ-ray (from the Fermi telescope), and VHE γ-ray (from Cherenkov telescopes) bands. For each source, the method involves best-fitting the spectral energy distribution from optical through HE γ-rays (the latter being largely unaffected by EBL attenuation as long as z ? 1) with a synchrotron self-Compton model. We extrapolate such best-fitting models into the VHE regime and assume they represent the BLLacs' intrinsic emission. Contrasting measured versus intrinsic emission leads to a determination of the γγ opacity to VHE photons. Using, for each given source, different states of emission will only improve the accuracy of the proposed method. We demonstrate this method using recent simultaneous multifrequency observations of the high-frequency-peaked BLLac object PKS2155-304 and discuss how similar observations can more accurately probe the EBL.