Constraints on Extragalactic Background Light using TeV Observations of BL Lacertae objects

Abstract

The extragalactic background light (EBL) in the IR to UV bands partly absorbs very high energy (VHE, $E \geq$ 100GeV) $γ-$ray photons travelling over cosmological distances via pair production. In this paper, to get stronger constraints on EBL, we use the deliberate selection of the EBL model and data of five BL Lacs with better statistics and the harder spectra to limit the EBL density and the radiation mechanism of BL Lacs. We constrain the upper limit of the EBL density by fitting the spectral energy distributions (SEDs) of TeV BL Lacs and find that our results are compatible with the published measurement, reaching 50 $\rm{nW m^{-2} sr^{-1}}$. We also obtain that the EBL is not necessarily transparent to high VHE photons. We fix the intrinsic spectral index $Γ_i$ of TeV BL Lacs as 1.0 and 1.5 under observation evidence and model assumption. Comparing the EBL density given by galaxy count and $Spitzer$ observations, we then obtain that 1ES 1101-232 has $Γ_i$ $\leq$ 1.0 and 1ES 0229+200 should have $Γ_i$ not harder than 1.0. We demonstrate that the common radiation assumption of BL Lacs, in which the $Γ_i$ is softer than 1.5, should be revisited. Furthermore, we propose that the upper EBL density could be given by fitting the hardest energy spectra of TeV BL Lacs.