Search for ultra-high energy photons through preshower effect with gamma-ray telescopes: Study of CTA-North efficiency

TL;DR

This study assesses the potential of CTA-North to detect ultra-high energy photon preshowers via Monte Carlo simulations, analyzing detection efficiency, event rates, and implications for astrophysical models and observatories.

Contribution

It introduces a detailed simulation-based evaluation of CTA-North's ability to identify preshower events from UHE photons, including efficiency and event rate predictions.

Findings

Detection efficiency for preshower events is quantified.

Expected event rates are extremely low under diffuse emission models.

Gamma-ray burst boosting could significantly increase detection prospects.

Abstract

We study the feasibility of detecting preshower initiated by ultra-high energy photons using Monte-Carlo simulations of nearly horizontal air showers for the example of the La Palma site of the Cherenkov Telescope Array. We investigate the efficiency of multivariate analysis in correctly identifying preshower events initiated by 40 EeV photons and cosmic-ray dominated background simulated in the energy range 10 TeV -- 10 EeV. The effective areas for such kind of events are also investigated and event rate predictions related to different ultra-high energy photons production models are presented. While the expected number of preshowers from diffuse emission of UHE photon for 30 hours of observation is estimated around $3.3\times10^{-5}$ based on the upper limits put by the Pierre Auger Observatory, this value is at the level of $2.7\times10^{-4}$ ($5.7\times 10^{-5}$) when considering the upper limits of the Pierre Auger Observatory (Telescope Array) on UHE photon point sources. However, UHE photon emission may undergo possible "boosting" due to gamma-ray burst, increasing the expected number of preshower events up to 0.17 and yielding a minimum required flux of $\sim 0.2$ $\mathrm{km^{-2}yr^{-1}}$ to obtain one preshower event, which is about a factor 10 higher than upper limits put by the Pierre Auger Observatory and Telescope Array (0.034 and 0.019 $\mathrm{km^{-2}yr^{-1}}$, respectively).