Possible causes of a rise with energy of the cosmic ray positron fraction

Abstract

Based on general considerations rather than model-dependent fits to specific scenarios, we argue that an increase with energy of the positron fraction in cosmic rays, suggested by several experiments at E > ∼ 7 GeV, most likely requires a primary source of electron-positron pairs. We discuss the possible alternatives, and find none of them plausible on astrophysical or particle physics grounds. Further observational ways to test different scenarios are discussed.

Synchrotron emission from young and nearby pulsars

“…[12,13]). This is in striking contrast with the predictions of models in which the cosmic positron flux is dominated by the secondary products of hadronic cosmic ray interactions [14,15]. More recently, the FGST collaboration has published their measurement of the cosmic ray electron (plus positron) spectrum between approximately 20 GeV and 1 TeV [16].…”

PAMELA and indirect dark matter searches

“…Concerning the positron fraction, the secondary production and propagation model considered in figure 13 is clearly not able to fully account for the experimental data. In particular the rise above 10 GeV is very difficult to reproduce with a pure secondary component without using an unrealistic soft electron spectrum [45], suggesting the existence of other primary sources [46].…”

“…Despite all these approximations, explaining the increasing ratio with contributions from standard secondary production would require large modifications of the experimentally established spectra of electrons, protons and helium nuclei [46], although some papers report an explanation of the observed excess in terms of nearby SNR [62] or of secondary production taking place in the same region where CR are being accelerated [63]. However, to distinguish among the different hypothesis a better knowledge of the standard production of electrons and positrons is required as well as of the mechanisms of their acceleration and transport in the galaxy.…”

Semi-annihilation of Dark Matter

“…Such an anomalous excess of the total e + + e − flux was confirmed by FERMI [43] and HESS [44,45], although the observed spectrum is softer than the peak reported by ATIC [46] and PPB-BETS [47]. These anomalies might be evidence for dark matter annihilation or decay in our galaxy [48,49,50,51,52], though astrophysical explanations have been put forward [53,54,55,56,57].…”