Constraints to Energy Spectra of Blazars based on Recent EBL Limits from Galaxy Counts

TL;DR

This study uses recent galaxy count data to set new constraints on the intrinsic energy spectra of distant TeV blazars, challenging existing models and assumptions about their spectral hardness.

Contribution

It introduces updated limits on the extragalactic background light based on galaxy counts, impacting models of blazar emission spectra.

Findings

Intrinsic spectra are at least as hard as index 1.28 ± 0.20.

Revised EBL limits imply blazars can have very hard spectra in the TeV range.

Challenges previous assumptions that spectra cannot be harder than 1.5.

Abstract

We combine the recent estimate of the contribution of galaxies to the 3.6 micron intensity of the extragalactic background light (EBL) with optical and near-infrared (IR) galaxy counts to set new limits on intrinsic spectra of some of the most distant TeV blazars 1ES 0229+200, 1ES 1218+30.4, and 1ES 1101-232, located at redshifts 0.1396, 0.182, and 0.186, respectively. The new lower limit on the 3.6 micron EBL intensity is significantly higher than the previous one set by the cumulative emission from resolved Spitzer galaxies. Correcting for attenuation by the revised EBL, we show that the differential spectral index of the intrinsic spectrum of the three blazars is 1.28 +- 0.20 or harder. These results present blazar emission models with the challenge of producing extremely hard intrinsic spectra in the sub-TeV to multi-TeV regime. These results also question the reliability of recently derived upper limits on the near-IR EBL intensity that are solely based on the assumption that intrinsic blazar spectra should not be harder than 1.5.