Constraints on the flux of primary cosmic-ray photons at energies E > 1018 eV from Yakutsk muon data
Abstract
Comparing the signals measured by the surface and underground scintillator detectors of the Yakutsk Extensive Air Shower Array, we place upper limits on the integral flux and the fraction of primary cosmic-ray photons with energies E > 1018 eV, E > 2*1018 eV and E > 4*1018 eV. The large collected statistics of the showers measured by large-area muon detectors provides a sensitivity to photon fractions < 10-2, thus achieving precision previously unreachable at ultra-high energies.
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