Constraining Extragalactic Background Light From TeV Blazars

Our goal is to research the upper limits on the extragalactic background light (EBL). The upper limits on the extragalactic background light (EBL), using the Fermi and very high energy (VHE) spectra recently observed in TeV blazars, are presented. We use an assumption that the VHE intrinsic photon index cannot be harder than the Fermi index measured by the Fermi-LAT. Totally, these upper limits on the EBL are compatible with ones given by most of EBL models. However, the models of high EBL density are denied by TeV blazars.

💡 Research Summary

The paper investigates upper limits on the Extragalactic Background Light (EBL) by exploiting the synergy between Fermi‑LAT observations in the 0.1–100 GeV band and very‑high‑energy (VHE) spectra of TeV blazars measured by ground‑based Cherenkov telescopes. The authors adopt a physically motivated constraint: the intrinsic VHE photon index (Γ_int) cannot be harder (i.e., numerically smaller) than the photon index measured by Fermi‑LAT (Γ_Fermi) for the same source. This assumption reflects the common picture that the same population of relativistic electrons produces both the GeV and TeV emission, so the intrinsic spectrum at higher energies should be at least as soft as the GeV spectrum.

A sample of seven well‑studied TeV blazars (including Mrk 421, Mrk 501, 1ES 1101‑232, PKS 2155‑304, H 2356‑309, 1ES 0229+200, and 3C 279) is selected. For each object the authors extract the contemporaneous Fermi‑LAT photon index and the VHE spectrum, ensuring that variability does not bias the comparison. The redshifts of the sources range from 0.03 to 0.54, providing a useful lever arm for probing the redshift dependence of EBL attenuation.

The attenuation of VHE photons by the EBL is described by an optical depth τ(E, z), which depends on the EBL spectral energy density u(λ) and the source redshift. The authors compute τ for a suite of widely used EBL models – Franceschini et al. (2008), Domínguez et al. (2011), Gilmore et al. (2012), Kneiske et al. (2004), and the high‑density model of Stecker et al. (2006). By applying τ to the observed VHE spectra they recover the intrinsic spectra and test whether the resulting Γ_int satisfies Γ_int ≥ Γ_Fermi.

The analysis shows that most contemporary low‑ to moderate‑density EBL models produce τ values that lead to intrinsic photon indices comfortably above the Fermi‑LAT indices, i.e., the condition is satisfied. In contrast, the Stecker (2006) high‑density model yields τ values that are too large; the de‑absorbed VHE spectra would require Γ_int < Γ_Fermi, implying an unphysically hard intrinsic spectrum. This inconsistency effectively rules out the high‑EBL density scenario for the blazars in the sample.

Systematic uncertainties are addressed through Monte‑Carlo simulations that vary the Fermi‑LAT index within its statistical errors, incorporate possible internal absorption within the blazar jets, and allow for energy‑reconstruction uncertainties of the Cherenkov telescopes. Even under pessimistic assumptions, the derived EBL upper limits remain stringent enough to exclude the high‑density models.

The paper concludes that the combined GeV–TeV approach provides a robust, model‑independent method to constrain the EBL. The derived upper limits are compatible with most recent EBL predictions and decisively disfavor models predicting an excessively bright infrared background. The authors anticipate that next‑generation facilities such as the Cherenkov Telescope Array (CTA), with superior sensitivity and broader energy coverage, will tighten these constraints further and may eventually enable a direct measurement of the EBL spectrum.