Studies of Extragalactic Background Light with TeV BL Lac Objects

TL;DR

This paper introduces a flexible optical depth method using SSC models to study the EBL via TeV BL Lac spectra, providing upper limits consistent with previous measurements and questioning some existing emission models.

Contribution

It develops a novel, model-dependent approach to constrain EBL density using SSC fits to TeV BL Lac spectra, incorporating different electron energy distributions.

Findings

Upper limits of EBL density around 30 nW m$^{-2}$ sr$^{-1}$

The method aligns with existing measurements of EBL density

Some BL Lacs may require alternative radiation models

Abstract

Very high energy (VHE; $E \geq$ 100GeV) $\gamma-$ray from cosmological distances are attenuated by the extragalactic background light (EBL) in the IR to UV band. By contrasting measured versus intrinsic emission, the EBL photon density can be derived. However, neither the intrinsic spectra nor the EBL is separately known~-~except their combined effect. Here we first present a flexible model-dependent optical depth method to study EBL by fitting the emission spectra of TeV BL Lacs objects via a one-zone leptonic synchrotron self-Compton model (SSC). We have little information about electron energy distributions (EEDs) in the jet, which is critically important to build spectral energy distributions (SEDs) in SSC scenario. Based on the current particle acceleration models, we use two types of EEDs, i,e., a power-law log-parabola (PLLP) EED and a broken power-law (BPL) EED to fit the observed spectra. We obtain that the upper limits of EBL density are around about 30 n W m$^{-2}$ sr$^{-1}$, which are similar to the published measurement. Furthermore, we propose an unprecedented method to test radiation mechanisms involved in TeV objects by simply comparing the reduced EBL density with the limit obtained by galaxy counts, and demonstrate that for some BL Lacs, at least, the one-zone SSC model should be reconsidered.