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

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.

20 pages, 12 figures. V2: important misprints corrected (there was a mismatch in numbers between the abstract and the main text, sorry). Version to appear in JETP in December 2013

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• The CTA Sensitivity to Lorentz-Violating Effects on the Gamma-Ray Horizon
• Constraints on the flux of $\sim (10^{16}-10^{17.5})$ eV cosmic photons from the EAS-MSU muon data
• Gamma-rays as probes of the Universe
• Full Monte-Carlo description of the Moscow State University Extensive Air Shower experiment
• Estimates of the cosmic gamma-ray flux at PeV to EeV energies from the EAS-MSU experiment data
• A method to calibrate the absolute energy scale of air showers with ultra-high energy photons