Abstract
The rise in the positron fraction, observed by PAMELA, Fermi-LAT and most recently by AMS-02, has created a lot of interest, fuelled by speculations about an origin in dark matter annihilation in the Galactic halo. However, other channels, e.g. antiprotons or gamma-rays, now severely constrain dark matter interpretations, thus requiring astrophysical sources of positrons. We have investigated the possibility that supernova remnants, the most likely sources of Galactic cosmic rays, can in fact also produce a hard spectrum of secondary positrons, by spallation and acceleration at the shock. This mechanism is guaranteed if hadronic CRs are present and would also lead to observable signatures in other secondary channels like the boron-to-carbon or antiproton-to-proton ratios. If such features were borne out by upcoming AMS-02 data, this would rule out other explanations.
Highlights
In propagation models of galactic cosmic rays (GCRs), the positron fraction, i.e. the ratio of positrons to the sum of electrons and positrons, is expected to fall at all energies
We have investigated the possibility that supernova remnants, the most likely sources of Galactic cosmic rays, can produce a hard spectrum of secondary positrons, by spallation and acceleration at the shock
We focus on a particular minimal and testable model [13, 14]: minimal in so far as no new class of CR sources is required; testable because the rise in the positron fraction is mirrored in similar features in other secondary-to-primary ratios, like B/C [15, 16, 17]
Summary
We have investigated the possibility that supernova remnants, the most likely sources of Galactic cosmic rays, can produce a hard spectrum of secondary positrons, by spallation and acceleration at the shock. 1. Introduction In propagation models of galactic cosmic rays (GCRs), the positron fraction, i.e. the ratio of positrons to the sum of electrons and positrons, is expected to fall at all energies. Published under licence by IOP Publishing Ltd can be determined from radio observations, the e ciency of electron-positron production, the spectral index and the cut-o↵ energy are mostly free parameters and are oftentimes fixed by hand to fit the observed lepton fluxes.
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