Cosmic-ray electron anisotropies as a tool to discriminate between exotic and astrophysical sources

Recent results from the PAMELA [1], ATIC [2], PPB BETS [3] and Fermi [4] collaborations extend the energy range in the e+, e− measurement up to unexplored energies in the hundreds of GeV range confirming the bump starting at about 10 GeV already suggested by HEAT [5] and AMS01 data [6]. This bump can be explained by the annihilation of dark matter (DM) in the context of exotic physics, or by nearby astrophysical sources such as pulsars. In order to discriminate between the competing models for primary positron production, the study of anisotropies, in addition to the spectrum determination, shows up as new tool for investigating the origin of the lepton excess. In this letter we calculate the contribution to the electron flux due to both the collection of all known gamma-ray pulsars (as listed in the ATNF catalogue) and by the annihilation of dark matter. In the latter case we consider that the excess can be attributed to a clumpy halo or a nearby sizeable dark matter clump. We address the problem of the electron anisotropy in both scenarios and estimate the prospect that a small dipole anisotropy might be found by the Fermi observatory.