What can we really learn from positron flux 'anomalies'?

Abstract

We present a critical analysis of the observational constraints on, and of the theoretical modelling of, aspects of cosmic ray (CR) generation and propagation in the Galaxy, which are relevant for the interpretation of recent positron and antiproton measurements. We give simple, analytic, model-independent expressions for the secondary flux, and an upper limit for the secondary e+ flux, obtained by neglecting e+ radiative losses, e+/(e++ e-) < 0.2 ± 0.1 up to ∼300 GeV. These expressions are completely determined by the rigidity-dependent grammage, which is measured from stable CR secondaries up to ∼150 GeV nucleon-1, and by nuclear cross-sections measured in the laboratory. and e+ measurements, available up to ∼100 GeV, are consistent with these estimates, implying that there is no need for new, non-secondary, or e+ sources. The radiative loss suppression factor of the e+ flux depends on the e+ propagation in the Galaxy, which is not understood theoretically. A rough, model-independent estimate of can be obtained at a single energy, e{open}∼ 20 GeV, from unstable secondary decay and is found to be consistent with e+ measurements, including the positron fraction measured by Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA). We show that specific detailed models, that agree with compositional CR data, agree with our simple expressions for the e+ and flux, and that the claims that the positron fraction measured by PAMELA requires new primary e+ sources are based on assumptions, that are not supported by observations. If PAMELA results are correct, they suggest that is slightly increasing with energy, which provides an interesting constraint on CR propagation models. We argue that measurements of the e+ to ratio are more useful for challenging secondary production models than the e+/(e++ e-) fraction. © 2010 The Authors. Journal compilation © 2010 RAS.