Galactic Scattering and Discrete Sources as Possible Origins of the Diffuse Galactic X-ray Background

Abstract

Cooke, Griffiths and Pounds1 have recently reported an excess in the diffuse X-ray background intensity observed at low galactic latitudes, compatible with a line source of radiation in the galactic plane. Wickramasinghe2 has put forward an explanation based on small angle scattering of the universal isotropic flux of X-rays by grains in the galaxy. The purpose of this article is seriously to question this hypothesis. Briefly, Wickramasinghe first argues that the albedo of the galaxy is near unity in the X-ray band, that is, the scattering cross-section of galactic material is quite high in certain directions, while the absorption cross-section is quite low. The second point in the argument is a derivation of a first order solution to the scattered component of the diffuse X-ray background. The answer obtained is where θ is the angle of the detector axis with the galactic plane, I1 is the intensity of once-scattered X-rays observed in the direction θ, and I is the intensity of extra-galactic diffuse X-rays. Hence the observed intensity in a direction close to the plane would be about double that observed at large θ.