# Extending the observation limits of Imaging Air Cherenkov Telescopes toward horizon

## TL;DR

This paper presents a method to extend the observation limits of Imaging Air Cherenkov Telescopes toward the horizon by calibrating Cherenkov light transmission at large zenith angles, significantly increasing collection area and event statistics.

## Contribution

It introduces an aperture photometry calibration technique enabling Cherenkov telescopes to observe air showers near the horizon, expanding their effective observation range.

## Key findings

- Collection area exceeds one square kilometer at large zenith angles.
- Method allows observations comparable to Cherenkov Telescope Array for selected sources.
- Enables study of high-energy gamma rays and cosmic ray origins.

## Abstract

Usually the Imaging Atmospheric Cherenkov Telescopes, used for the ground-based gamma-ray astronomy in the very high energy range 50 GeV - 50 TeV, perform air shower observations till the zenith angle of ~60 deg. Beyond that limit the column density of air increases rapidly and the Cherenkov light absorption starts playing a major role. Absence of a proper calibration method of light transmission restrained researchers performing regular measurements under zenith angles >>60 deg. We extend the observation of air showers in Cherenkov light till almost the horizon. We use an aperture photometry technique for calibrating the Cherenkov light transmission in atmosphere during observations under very large zenith angles. Along with longer in time observations of a given source, this observation technique allows one to strongly increase the collection area and the event statistics of Cherenkov telescopes for the very high energy part of the spectrum. Study of the spectra of the highest energy gamma rays from a handful of candidate sources can provide a clue for the origin of the galactic cosmic rays. We show that MAGIC very large zenith angle observations yield a collection area in excess of a square kilometer. For selected sources this is becoming comparable with the target collection area anticipated with the Cherenkov Telescope Array.

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Source: https://tomesphere.com/paper/1903.04989