Energetic gamma ray experiment telescope high‐energy gamma ray observations of the Moon and quiet Sun

High‐energy gamma radiation has been seen from the Moon as it passed through the large field of view of the energetic gamma ray experiment telescope (EGRET) on the Compton Gamma Ray Observatory a number of times between mid‐1991 and the end of 1994. The average lunar flux is (4.7 ± 0.7) × 10 −7 photons ( E > 100 MeV) cm −2 s −1 . The observed energy spectrum is consistent with a model of gamma ray production by cosmic ray interactions with the lunar surface, and the flux varies as expected with the solar cycle. Although the same types of cosmic ray interactions that produce the lunar gamma rays may occur on the Sun, EGRET has not detected the quiet Sun. The 95% confidence upper limit on the flux above 100 MeV, 2.0 × 10 −7 photons cm −2 s −1 , lies well below the lunar gamma ray flux, as expected.