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/)

Pion decay gamma rays have long been recognized as a unique signature of hadronic cosmic rays and their interactions with the interstellar medium. We present a model-independent way of constraining this signal with observations of the Galactic Plane in diffuse gamma rays. We combine detections by the EGRET instrument at GeV energies and the Milagro Cherenkov detector at TeV energies with upper limits from KASCADE and CASA-MIA ground arrays at PeV energies. Such a long "lever arm", spanning at least six orders of magnitude in energy, reveals a "TeV excess" in the diffuse Galactic Plane gamma-ray spectrum. While the origin of this excess is unknown, it likely implies also enhanced TeV neutrino fluxes, significantly improving the prospects for their detection. We show that unresolved point sources are a possible source of the TeV excess. In fact, the spectra of the unidentified EGRET sources in the Milagro region must break between ~10 GeV and ~1 TeV to avoid strongly overshooting the Milagro measurement; this may have important implications for cosmic-ray acceleration. Finally, we use our approach to examine the recent suggestion that dark-matter annihilation may account for the observed excess in diffuse Galactic gamma-rays detected by EGRET at energies above 1 GeV. Within our model-independent approach, current data cannot rule this possibility in or out; however we point out how a long "lever arm" can be used to constrain the pionic gamma-ray component and in turn limit the "GeV excess" and its possible sources. Experiments such as HESS and MAGIC, and the upcoming VERITAS and GLAST, should be able to finally disentangle the main sources of the Galactic gamma rays.

Reference: T. Prodanović, B. D. Fields & J. F. Beacom 2006, Astropart. Phys. submitted (get from astro-ph/)