# Very high-energy constraints on the Infrared Extragalactic Background Light

## TL;DR

This paper explores an indirect method using gamma-ray observations to constrain the infrared extragalactic background light (EBL) by analyzing pair-production effects, focusing on local VHE sources with upcoming telescopes.

## Contribution

It proposes a novel approach to measure the IR EBL from 3 to 100 micrometers using gamma-ray interactions, bypassing foreground contamination issues.

## Key findings

- Local VHE sources can help constrain IR EBL up to 100 μm.
- Distant blazars are less effective for EBL constraints due to their distance.
- Upcoming gamma-ray observatories are suitable for this indirect measurement.

## Abstract

Context. Measurements of the Extragalactic Background Light (EBL) are a fundamental source of information on the collective emission of cosmic sources. Aims. At infrared wavelengths, however, these measurements are precluded by the overwhelming dominance from Interplanetary Dust emission and the Galactic infrared foreground. Only at $\lambda > 300 \ \mu$m, where the foregrounds are minimal, has the Infrared EBL (IR EBL) been inferred from analysis of the COBE maps. The present paper aims to assess the possibility of evaluating the IR EBL from a few $\mu$m up to the peak of the emission at >100 $\mu$m using an indirect method that avoids the foreground problem. Methods. To this purpose we exploit the effect of pair-production from gamma-gamma interaction by considering the highest energy photons emitted by extragalactic sources and their interaction with the IR EBL photons. We simulate observations of a variety of low redshift emitters with the forthcoming Imaging Atmospheric Cherenkov Telescope (IACT) arrays (CTA in particular) and water Cherenkov observatories (LHAASO, HAWC, SWGO) to assess their suitability to constrain the EBL at such long wavelengths. Results. We find that, even under the most extremely favorable conditions of huge emission flares, extremely high-energy emitting blazars are not very useful for our purpose because they are much too distant (>100 Mpc the nearest ones, MKN 501 and MKN 421). Observations of more local Very High Energy (VHE) emitting AGNs, like low-redshift radio galaxies (M87, IC 310, Centaurus A), are better suited and will potentially allow us to constrain the EBL up to $\lambda \simeq 100\ \mu$m.