Isotropic Gamma Radiation and the Metagalactic Cosmic Ray Intensity

Abstract

DATA from the OSO-3 high energy gamma ray experiment have indicated an apparent isotropic background flux of gamma rays with energies greater than 100 MeV (ref. 1). The intensity of this flux has been found to be less than or equal to (1.1 ± 0.2) × 10⁻⁴ cm⁻² s⁻¹ sr⁻¹. Clark et al.¹ have pointed out that this flux may be compatible with an extrapolation of the observed X-ray spectrum below 1 MeV if the isotropic X-rays continue to follow a power-law spectral form. The explanation most usually considered for this power-law spectrum is that the X-rays may be generated by the interaction of metagalactic cosmic ray electrons with the universal thermal radiation^2,3. (While the OSO-3 data are compatible with an extrapolation of the X-ray spectrum, they may indicate a gamma ray flux greater than this extrapolation. Above 100 keV, the differential X-ray spectrum is of the form⁴, I(E_γ) ≅ 1.5 × 10⁻², E_γ^−2.3 cm⁻² MeV⁻¹ s⁻¹ sr⁻¹ with E_γ in MeV. Such a power law would predict an integral flux above 100 MeV of 2.9 × 10⁻⁵ cm⁻² s⁻¹ sr⁻¹, a factor of 4 below the OSO-3 value.)