Galactic secondary positron flux at the Earth

Context. Current measurements of the positron cosmic rays exhibit a bump around 10 GeV which is still hardly explained by standard secondary astrophysical processes, i.e. spallation of cosmic rays off the interstellar gas. Many scenarios have been invoked as potential solutions to this excess, among them being some additional primary positrons originating from dark matter annihilation in the Galaxy. Aims. While the PAMELA satellite is about to yield much more precise data, it is of paramount importance to theoretically constrain the expected secondary positron flux before any putative interpretation of the observations. Moskalenko and Strong (1998) provided a precise estimate some years ago, but using rather old parameterizations for the nuclear cross sections, and without giving the theoretical uncertainties affecting their predictions. Methods. We reestimate the secondary positron flux by using and comparing different up-to-date nuclear cross sections and, and by considering an independent model of cosmic ray propagation. We also carefully study the origins of the theoretical uncertainties. Results. We find the secondary positron flux to lie in the range sketched by the current observations, associated with theoretical uncertainties of about one order of magnitude. We also discuss the positron fraction issue, and show that our predictions of the secondary positron flux may be perfectly consistent with what has long been called an excess. Conclusions. We give some new insights which may help to find trails to explain the positron excess with or without new physics. PAMELA observations and the forthcoming AMS-02 mission will soon allow much better constraints on the cosmic ray transport parameters, and are likely to drastically reduce those uncertainties.