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

Galactic cosmic rays consist of protons, electrons and ions, most of which are believed to be accelerated to relativistic speeds in supernova remnants(1-3). All components of the cosmic rays show an intensity that decreases as a power law with increasing energy ( for example as E 22.7). Electrons in particular lose energy rapidly through synchrotron and inverse Compton processes, resulting in a relatively short lifetime ( about 10 5 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 pulsar(5) or micro-quasar(6)) that accelerates electrons to those energies, or the electrons could arise from the annihilation of dark matter particles ( such as a Kaluza-Klein particle(7) with a mass of about 620 GeV).