Escape of Secondary Cosmic-Ray Positrons Produced in a Supernova Remnant

TL;DR

This paper investigates the acceleration and escape of secondary positrons from supernova remnants, challenging previous models by showing that their spectra are softer than primary cosmic rays, thus questioning the explanation of the positron excess.

Contribution

It provides a new calculation of secondary positron spectra from SNRs, demonstrating that these spectra are softer than primary cosmic rays, contrary to earlier assumptions.

Findings

Secondary CR positron spectra are softer than primary CR spectra.

The positron excess cannot be explained by secondary production in SNRs.

Previous models overestimated the hardness of secondary spectra.

Abstract

We discuss the acceleration and escape of secondary particles, especially positrons produced by hadronic interactions in a supernova remnant (SNR) shock. During the shock acceleration, protons would interact with ambient gas and produce charged secondary particles, which would also be accelerated in a SNR and injected into the interstellar medium as cosmic-rays (CRs). Some previous studies showed that the resulting positron spectrum at the SNR shock is harder than the primary proton spectrum, and proposed that the positron excess observed by PAMELA can be explained with this process. We calculate the energy spectra of CR protons and secondary CR positrons running away from the SNR into the interstellar medium according to the phenomenological model of energy-dependent CR escape. We show that, on the contrary to the results presented previously, the observed spectra of secondary CR particles generated in SNRs would be softer than those of primary CR particles, which means that the rise in the positron fraction cannot be reproduced by this model.