Cosmic-ray electron spectrum

Last fall, the ATIC balloon collaboration reported a tantalizing peak near 500 GeV in its measured spectrum of high-energy cosmic-ray electrons (Physics Today, January 2009, page 16). The peak suggested that 500-GeV dark-matter particles of the kind predicted by extra-dimensional extensions of standard particle theory might be annihilating each other in nearby accumulations of dark matter to produce energetic electron-positron pairs. Now NASA's recently launched Fermi Gamma-ray Space Telescope has measured the electron spectrum out to 1 TeV with much higher statistics (see the figure). Fermi is designed primarily to record high-energy gammas, but it can also detect electrons.  The Fermi data show no narrow spectral feature near 500 GeV, nor anywhere else. But above 100 GeV, they do exhibit a growing excess over the predictions of a conventional diffusive model of electrons from very distant astrophysical sources. The positron spectrum measured by the orbiting PAMELA magnetic spectrometer showed a similar excess above 10 GeV. (Neither ATIC nor Fermi can distinguish electrons from the much rarer positrons.) Taken together, the Fermi and PAMELA results suggest that our local galactic neighborhood harbors either an undiscovered astrophysical electron-positron source (most likely a pulsar) or a dense concentration of unidentified, heavy dark-matter particles. Much should be revealed when the Fermi collaboration reports electron spectral data beyond 1 TeV and the spectrum of the diffuse gamma-ray background out to 1 TeV. (A. A. Abdo et al., Fermi collaboration, Phys. Rev. Lett. 102, 181101, 2009; O. Adriani, PAMELA collaboration, Nature 458, 607, 2009.) —Bertram Schwarzschild