# Investigation of Energy Spectrum of EGRET Gamma-ray Sources by an Extensive Air Shower Experiment

## TL;DR

This study monitored ultra-high-energy gamma-ray sources over five years using an air shower array, analyzing their spectra and setting upper limits on fluxes for selected EGRET sources.

## Contribution

It provides new measurements and flux estimates of UHE gamma-ray sources from EGRET catalog using extensive air shower data and analysis methods.

## Key findings

- Upper limits for 10 EGRET sources were established.
- Fluxes of sources were estimated in the UHE gamma-ray range.
- Gamma-ray spectra were investigated in the ultra-high-energy domain.

## Abstract

Ultra-High-Energy (UHE) ($E>100 $TeV) Extensive Air Showers (EASs) have been monitored for a period of five years (1997-2003), using a small array of scintillation detectors in Tehran, Iran. The data have been analyzed to take in to account of the dependence of source counts on zenith angle. Because of varying thickness of the overlaying atmosphere, the shower count rate is extremely dependent on zenith angle. During a calendar year different sources come in the field of view of the array at varying zenith angles and have different effective observation time equivalent to zenith in a day. High energy gamma-ray sources from the EGRET third catalogue where observed and the data were analyzed using an excess method. Upper limits were obtained for 10 EGRET sources. Then we investigated the EAS event rates for these 10 sources and obtained a flux for each of them using parameters of our experiment results and simulations. Finally we investigated the gamma-ray spectrum in the UHE range using these fluxes with reported fluxes of the EGRET sources.