Constraints on extragalactic background light using TeV observations of BL Lacertae objects

Abstract

ABSTRACT The extragalactic background light (EBL) in the infrared to ultraviolet bands partly absorbs very high energy (VHE; E ≥ 100 GeV) 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 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 VHE photons. We fix the intrinsic spectral index Γi 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 Γi ≤ 1.0 and 1ES 0229+200 should have Γi not harder than 1.0. We demonstrate that the common radiation assumption of BL Lacs, in which the Γi 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.