We agree with Martin Israel that, to resolve the discrepancy, both experiments need to work to be sure that the systematic uncertainties are correct. Still, we conclude that the evidence for a prominent feature in the spectrum has been cast in serious doubt. And since our Viewpoint appeared, the HESS team has released a measurement of the flux in the energy range 400 GeV – 5 TeV , overlapping with the Fermi result. HESS and Fermi agree (on the lack of a feature), each with very high statistical significance. It is possible that both HESS and Fermi have treated their systematics incorrectly, causing each to miss the feature observed by ATIC, but the evidence at this point is leaning in the direction of Fermi and now HESS: there is no prominent feature in the 400–800 GeV range. As Israel points out, the most crucial systematic uncertainties involve the behavior of the calorimeters, and those of Fermi and ATIC have relative strengths and weaknesses. Experiments with high statistics are generally better able to probe unforeseen systematic uncertainties than are those with low statistics. We, along with Israel, look forward to having these discrepancies resolved by the experiments in question.
- F. Aharonian et al., arXiv:0905.0105 (2009).