High-energy γ rays of cosmic origin

Abstract

A stack of nuclear emulsions flown over Hyderabad (vertical geomagnetic cutoff rigidity 16.9 GV), India, under 6 g/cm² of residual air has been used to set upper limits to the flux of γ rays of cosmic origin. The integral flux values obtained are 13 × 10⁻⁴, 6 × 10⁻⁴, 2.6 × 10⁻⁴, 1.4 × 10⁻⁴, 0.6 × 10⁻⁴, and 0.3 × 10⁻⁴ photons (em² s sr)⁻¹ for energies greater than 50 MeV, 100 MeV, 2 GeV, 5 GeV, 10 GeV, and 50 GeV respectively. Using these data and those of Kraushaar et al. (1965), it seems unlikely that cosmic-ray collisions in meta-galactic space can be an important source of electrons needed to explain the finite flux of cosmic X rays as due to the inverse Compton scattering of these electrons with photons of the universal blackbody radiation at 3 °K; such collisions would predict a flux of γ rays much in excess of the upper limits obtained in the energy region 50–100 MeV.