Estimate of the fraction of primary photons in the cosmic-ray flux at energies ~10^17 eV from the EAS-MSU experiment data

TL;DR

This study reanalyzed EAS-MSU data and confirmed the presence of primary gamma rays at around 10^17 eV, estimating their fraction in cosmic rays and examining their origins.

Contribution

It provides the first evidence for a nonzero flux of primary cosmic gamma rays at ~10^17 eV using archival data, with a new estimate of their fraction in cosmic rays.

Findings

Estimated gamma-ray fraction: ~0.43% of cosmic rays above 5.4×10^16 eV.

Detected gamma-ray flux intensity: ~1.2×10^{-16} cm^{-2} s^{-1} sr^{-1}.

Arrival directions do not indicate a specific origin mechanism.

Abstract

We reanalyze archival EAS-MSU data in order to search for events with anomalously low content of muons with energies E_mu>10 GeV in extensive air showers with number of particles N_e>2*10^7. We confirm the first evidence for nonzero flux of primary cosmic gamma rays at energies E~10^17 eV. The estimated fraction of primary gamma rays in the flux of cosmic particles with energies E>5.4*10^16 eV is (0.43 +0.12-0.11)%, which corresponds to the intensity of (1.2 +0.4-0.3)*10^{-16} cm^{-2} s^{-1} sr^{-1}. The study of arrival directions does not favour any particular mechanism of the origin of the photon-like events.