Constraining the Hadronic Gamma-Rays

Diffuse gamma rays probe the highest-energy processes at the largest scales. We derived a model-independent constraints on the hadronic contribution to the Galactic and extragalactic gamma-ray spectra from the EGRET data. The hadronic component is dominated by emission from neutral pions, with a characteristic spectrum symmetric about pion rest-mass. We exploit the well-defined properties of the pion decay spectrum to quantify the maximum pionic fraction of the observed gamma-ray intensity. We find that the Galactic spectrum above 30 MeV can be at most about 50% pionic. The maximum pionic contribution to the extragalactic spectrum is energy dependent; it also depends on the redshift range over which the sources are distributed, ranging from as low as about 20% for pions generated very recently, to as much as 90% if the pions are generated around redshift 10.

Reference: Prodanović, T. &  Fields, B. D. 2004,  Astropart.Phys. 21, 627 (get from astro-ph/)