Synopsis: Do cosmic rays account for all the gamma rays in diffuse galactic radiation?

New data are inconsistent with previous measurements that showed an unexpected excess of diffuse gamma-ray emission in the Galaxy.
Synopsis figure
Illustration: Fermi LAT Collaboration, Phys. Rev. Lett. (2009)

The interactions of cosmic rays with the galactic interstellar gas and radiation field result in the diffuse emission of gamma rays. If these cosmic ray interactions are the only source of such diffuse emission, then models can be constructed which fit both the cosmic ray spectra and measurements of the diffuse gamma radiation. However, several years ago, the Energetic Gamma-Ray Experiment Telescope (EGRET) at the Compton Gamma-Ray Observatory reported [1] measurements of the diffuse galactic gamma-ray emission that were larger than what the models expected. A variety of explanations for this discrepancy have been offered, with particular excitement focused on the possibility that it is the annihilation of dark matter particles that produces the excess gamma rays.

Now, new data on the diffuse galactic gamma-ray emission from the Large Area Telescope (LAT) on the Fermi Gamma-Ray Space Telescope are reported in Physical Review Letters. The LAT instrument represents a significant improvement in sensitivity and resolution compared with EGRET. The Fermi LAT Collaboration reports that both their results and the measured cosmic-ray spectra are consistent with a model of diffuse galactic gamma-ray emission—which means that the EGRET gamma-ray excess is not confirmed. – Stanley Brown

[1] S. D. Hunter et al., Astrophys. J. 481, 205 (1997).


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