The pionic contribution to diffuse γ-rays: upper limits

Abstract

Diffuse gamma-rays probe the highest-energy processes at the largest scales. Here we derive model-independent constraints on the hadronic contribution to the Galactic and extragalactic gamma-ray spectra at in the energy range 50 MeV less than or similar to E-gamma less than or similar to 10 GeV. The hadronic component is dominated by emission from neutral pions, with a characteristic spectrum symmetric about m(pi0)/2. 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 almost about 50% pionic. The maximum picnic 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. The implications of these constraints for models of gamma-ray and neutrino emission are briefly discussed. (C) 2004 Elsevier B.V. All rights reserved.