Origin of Galactic Gamma-rays

Abstract

LAST year saw the birth of observational gamma-ray astronomy with the detection of cosmic gamma-rays with energies above 100 MeV energy by Clark et al.¹. Their results indicated that cosmic gamma-radiation is strongly anisotropic, being most intense in the galactic plane and particularly at the galactic centre. There seems to be a background component of isotropic gamma-radiation possibly of extragalactic origin (which I have already discussed in ref. 2) having a magnitude of ∼ 1×10⁻⁴ cm⁻² s⁻¹ sr⁻¹ Superimposed on this background there seems to be a component from the galactic disk, having a line intensity of 1–2 × 10⁻⁴ cm⁻² s⁻¹ rad⁻¹. Gamma-rays coming from the region of the galactic centre have a line intensity of 4–5 × 10⁻⁴ cm⁻² s⁻¹ rad⁻¹. Clark et al.¹ suggested that these fluxes, being an order of magnitude higher than those predicted from various diffuse production mechanisms, may originate mainly in unresolved discrete sources (a suggestion which was further explored by Ögelmann³). They were careful to point out, however, that large amounts of undetected hydrogen may also account for these fluxes.