An excess of cosmic ray electrons at energies of 300–800 GeV

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Abstract

Galactic cosmic rays consist of protons, electrons and ions, most of which are believed to be accelerated to relativistic speeds in supernova remnants1,2,3. All components of the cosmic rays show an intensity that decreases as a power law with increasing energy (for example as E-2.7). Electrons in particular lose energy rapidly through synchrotron and inverse Compton processes, resulting in a relatively short lifetime (about 105 years) and a rapidly falling intensity, which raises the possibility of seeing the contribution from individual nearby sources (less than one kiloparsec away)4. Here we report an excess of galactic cosmic-ray electrons at energies of 300–800 GeV, which indicates a nearby source of energetic electrons. Such a source could be an unseen astrophysical object (such as a pulsar5 or micro-quasar6) that accelerates electrons to those energies, or the electrons could arise from the annihilation of dark matter particles (such as a Kaluza–Klein particle7 with a mass of about 620 GeV).

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Figure 1: Separation of electrons from protons in the ATIC instrument.
Figure 2: ATIC-1 and ATIC-2 spectra at balloon altitude, showing good agreement with each other.
Figure 3: ATIC results showing agreement with previous data at lower energy and with the imaging calorimeter PPB-BETS at higher energy.
Figure 4: Assuming an annihilation signature of Kaluza–Klein dark matter, all the data can be reproduced.

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Acknowledgements

This research was supported in the USA by NASA, in Russia by the Russian Foundation for Basic Research, and in China by the National Natural Science Foundation. The help of the NASA BPO and CSBF during balloon flights and the US NSF and RPSC for Antarctica operations is gratefully acknowledged.

Author Contributions Each institution in the ATIC collaboration assumed particular roles, each of which provided important contributions to this paper. In particular, the Max Planck Institute and Purple Mountain Observatory (PMO) developed the electron observation techniques. Marshall Space Flight Center (MSFC), Moscow State University (MSU) and Louisiana State University (LSU) developed the hardware which was integrated at LSU, who also led the flight team. The University of Maryland (UMD) and MSU performed instrument simulations and processed the data. PMO led the electron analysis of the flight data and LSU and PMO prepared the manuscript.

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Correspondence to J. P. Wefel.

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Chang, J., Adams, J., Ahn, H. et al. An excess of cosmic ray electrons at energies of 300–800 GeV. Nature 456, 362–365 (2008) doi:10.1038/nature07477

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