The Galactic Diffuse Gamma-ray Spectrum from Cosmic-ray Proton Interactions

Abstract

A new calculation of the Galactic diffuse gamma-ray spectrum from the decay of secondary particles produced by interactions of cosmic-ray protons with interstellar matter is presented. The calculation utilizes the modern Monte Carlo event generators, Hadrin, Fritiof and Pythia, which simulate high-energy proton-proton collisions and are widely used in studies of nuclear and particle physics, in addition to scaling calculation. This study is motivated by the result on the Galactic diffuse gamma-ray flux observed by the EGRET detector on the Compton Gamma-ray Observatory, which indicates an excess above about 1 GeV of the observed intensity compared with a model prediction. The prediction is based on cosmic-ray interactions with interstellar matter, in which secondary pion productions are treated by a simple model. With the improved interaction model used here, however, the diffuse gamma-ray flux agrees rather well with previous calculations within uncertainties, which mainly come from the unobservable demodulated cosmic-ray spectrum in interstellar space. As a possible solution to the excess flux, flatter spectra of cosmic-ray protons have been tested and we found that the power-law spectrum with an index of about $-(2.4\sim 2.5)$ gives a better fit to the EGRET data, though the spectrum is not explained completely.

To appear in Astrophysical Journal 478 (March 20, 1997). LaTeX with AASTeX Macros v4.0, 24 pages (including 14 eps figures)

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