Estimates of the cosmic gamma-ray flux at PeV to EeV energies from the EAS-MSU experiment data

Abstract

Archival EAS-MSU data are searched for anomalous muonless events which may be caused by primary gamma rays with energies between 10¹⁵ and 10¹⁸ eV. We consider a refined sample of high-quality data and confirm the previously reported detection of a nonzero gamma-ray flux at ∼5 × 10¹⁶ eV with a similar flux value but at somewhat lower statistical significance, corresponding to a depletion of the sample. We present upper limits on the flux below and above these energies, including the first constraints in the range 10^16.5–10^17.5 eV never studied by any other experiment.