Upper Limit on Gamma-Ray Flux above 10~19 eV Estimated by the Akeno Giant Air Shower Array Experiment

摘要

The origin of the highest energy cosmic rays (≥10~20 eV) is not well understood. Interesting models called "top-down" scenarios have been proposed to explain the origin. The γ-ray flux in ultrahigh-energy cosmic rays is a key parameter for giving constraints on such models. To study the properties of γ-ray showers, we carry out simulation studies that take into account both the Landau-Pomeranchuk-Migdal effect and electromagnetic interactions in the geomagnetic field. Based on an analysis of muons in air showers observed by the Akeno Giant Air Shower Array, the upper limits on the γ-ray flux are estimated to be 28/100 above 10~19 eV and 67/100 above 10~9.5 eV in the observed air showers at a confidence level of 95/100.