Cosmic-ray positrons from millisecond pulsars

TL;DR

This paper models the contribution of millisecond pulsars to cosmic-ray positrons and electrons, finding they modestly contribute around tens of GeV and possibly significantly at TeV energies, impacting cosmic-ray origin theories.

Contribution

It introduces a detailed population synthesis and pair spectrum simulation for millisecond pulsars, including effects of magnetic field offsets and intrabinary shocks, to estimate their cosmic-ray contributions.

Findings

Millisecond pulsars modestly contribute to positron flux around tens of GeV.

Black widow and redback systems could contribute significantly at TeV energies.

The contribution depends on magnetic field offsets and system-specific acceleration processes.

Abstract

Observations by the Fermi Large Area Telescope of gamma-ray millisecond pulsar light curves imply copious pair production in their magnetospheres, and not exclusively in those of younger pulsars. Such pair cascades may be a primary source of Galactic electrons and positrons, contributing to the observed enhancement in positron flux above ~10 GeV. Fermi has also uncovered many new millisecond pulsars, impacting Galactic stellar population models. We investigate the contribution of Galactic millisecond pulsars to the flux of terrestrial cosmic-ray electrons and positrons. Our population synthesis code predicts the source properties of present-day millisecond pulsars. We simulate their pair spectra invoking an offset-dipole magnetic field. We also consider positrons and electrons that have been further accelerated to energies of several TeV by strong intrabinary shocks in black widow and redback systems. Since millisecond pulsars are not surrounded by pulsar wind nebulae or supernova shells, we assume that the pairs freely escape and undergo losses only in the intergalactic medium. We compute the transported pair spectra at Earth, following their diffusion and energy loss through the Galaxy. The predicted particle flux increases for non-zero offsets of the magnetic polar caps. Pair cascades from the magnetospheres of millisecond pulsars are only modest contributors around a few tens of GeV to the lepton fluxes measured by AMS-02, PAMELA, and Fermi, after which this component cuts off. The contribution by black widows and redbacks may, however, reach levels of a few tens of percent at tens of TeV, depending on model parameters.