[0705.4311] The Likely Cause of the EGRET GeV Anomaly and its Implications

Authors:  F. W. Stecker, S. D. Hunter, D. A. Kniffen (NASA/GSFC)
Abstract:  Analysis of data from the EGRET gamma-ray detector on the Compton Gamma Ray Observatory indicated an anomaly in the form of an excess diffuse galactic flux at GeV energies over that which was theoretically predicted. Various explanations for this anomaly have been put forth, including the invocation of supersymmetric dark matter annihilation. We reexamine these explanations here, including a new discussion of the possible systematic errors in the sensitivity determination of the EGRET detector. We conclude that the most likely explanation of the EGRET ``GeV anomaly'' was an error in the estimation of the of the EGRET sensitivity at energies above ~1 GeV. We give reasons why such a situation could have occurred. We find evidence from our new all-sky analysis which is inconsistent with the assumption that the anomaly can be a signal of supersymmetric dark matter annihilation. We also reconfirm the original results of the EGRET team on the extragalactic gamma-ray background spectrum. There are important implications of our analysis for the upcoming Gamma Ray Large Area Telescope (GLAST) mission.
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Syksy Rasanen
Posts: 119
Joined: March 02 2005
Affiliation: University of Helsinki

[0705.4311] The Likely Cause of the EGRET GeV Anomaly and i

Post by Syksy Rasanen » May 31 2007

What do people familiar with the EGRET anomaly think of this paper? Is it indeed likely that it would be just a calibration error, as the authors claim?

Andy Strong
Posts: 5
Joined: June 17 2007
Affiliation: Max-Planck-Institut für extraterrestrische Physik

[0705.4311] The Likely Cause of the EGRET GeV Anomaly and

Post by Andy Strong » June 18 2007

At present we really don't know for sure, but up to now it was always stated as unlikely by the instrumentalists. In this paper the view is taken that a particular diffuse emission model is reliable and hence EGRET can be renormalized to it. However no physical basis is given for the instrumental effect, as the authors admit. In fact a recent paper (0706.0503), which does an extensive Monte-Carlo of EGRET, actually finds the excess is underestimated. There are several astrophysical explanations - as referenced in their paper - including, plausibly, cosmic-ray spectra different from local in the Galaxy at large (while their's is based just on the local cosmic-ray spectrum). Another possibility is populations of unresolved sources (astro-ph/0609359). Still, with GLAST to be launched early next year, it is probably best just to wait and see. A more extensive discussion can be found at http://galprop.stanford.edu/forum/viewtopic.php?t=43 .

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