# Letter on: Cosmic light matter probes heavy dark matter

Physics 2, 53
A Viewpoint commentary discussing recent results from the Fermi Gamma-Ray Telescope on cosmic rays from dark matter may have dismissed an unusual spectral feature too soon; the authors respond.

The Viewpoint by Bruce Winstein and Kathryn M. Zurek [1] highlights the recent measurement of high-energy cosmic-ray electrons by the Fermi Gamma-Ray Space Telescope [2], in the context of two other recent electron measurements—the PAMELA spacecraft [3], which operates at lower energy than Fermi but distinguishes electrons from positrons, and the ATIC balloon-borne instrument [4], which presents results in the same energy interval as Fermi and like Fermi does not distinguish electrons from positrons. Their commentary is valuable for its description of the astrophysical significance of these cosmic-ray electron results.

However, in my view Winstein and Zurek are in error when they refer to the distinct difference between the Fermi and ATIC results and state unequivocally “… the large ATIC excess … is ruled out.” To be sure, the Fermi results have better statistical precision than the ATIC results, but the difficult part of this high-energy electron measurement is in the systematic uncertainties—distinguishing electrons from protons, which are $∼104$ times more abundant at these energies, and accurately measuring electron energy in a steeply falling energy spectrum (flux proportional to $E-n$ with $n∼3$). Both the ATIC and the Fermi scientists have carefully examined their systematic uncertainties and both have a great deal of confidence in their results. Of course, the discrepancy between the two makes clear that one or the other (if not both) contains some error, but we cannot be certain right now where the error lies.

Neither the Fermi telescope nor the ATIC instrument was designed primarily for the electron measurements reported here. The Fermi telescope was optimized for gamma rays up to 300 GeV; it does not have the depth of the ATIC instrument for fully containing the higher-energy electron showers. The ATIC instrument was optimized for high-energy cosmic-ray nuclei; it does not have the fine spatial resolution for visualizing the beginning of the shower that the Fermi telescope has.

It is clear to me, and I believe it is also clear to both sets of scientists, that both groups have work to do to understand and resolve their differences. The Fermi results may turn out to be correct, but in my view it is premature to declare unequivocally that Fermi is right and ATIC is wrong.

## References

1. B. Winstein and K. M. Zurek, Physics 2, 37 (2009)
2. A. A. Abdo et al., Phys. Rev. Lett. 102, 181101 (2009)
3. O. Adriani et al., Nature 458, 607 (2009);
4. J. Chang et al., Nature 456, 362 (2008)

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