The Pionic Contribution to Diffuse Gamma 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 in the energy range 50 MeV < E_gamma < 10 GeV. The hadronic component is dominated by emission from neutral pions, with a characteristic spectrum symmetric about m_{pi^0}/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 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. The implications of these constraints for models of gamma-ray and neutrino emission are briefly discussed.

14 pages, 4 figures, AASTeX. Comments welcome

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• Cosmic rays, lithium abundance and excess entropy in galaxy clusters
• Can Galactic Cosmic Rays Account for Solar 6Li Without Overproducing Gamma Rays?
• Extragalactic Gamma-ray Background from Star-forming Galaxies: Will Empirical Scalings Suffice?
• The Gamma-Ray Background Constrains the Origins of the Radio and X-Ray Backgrounds
• Diffuse Pionic Gamma-Ray Emission from Large Scale Structures in the FERMI Era
• Neutrino Constrains to the Diffuse Gamma-Ray Emission from Accretion Shocks