The millisecond pulsar contribution to the rising positron fraction

TL;DR

This paper models the contribution of millisecond pulsars and their binary systems to the Galactic cosmic-ray positron flux, finding modest contributions at tens of GeV and significant potential from binary systems at higher energies.

Contribution

It introduces a comprehensive model including MSP populations, magnetic field offsets, and binary system acceleration to estimate their cosmic-ray contributions.

Findings

MSP pair cascades modestly contribute around tens of GeV.

Binary systems like BW and RB could account for up to a few tens of percent of flux up to TeV energies.

Future observations can better constrain MSP and binary system contributions.

Abstract

Pair cascades from millisecond pulsars (MSPs) may be a primary source of Galactic electrons and positrons that contribute to the increase in positron flux above 10 GeV as observed by PAMELA and AMS-02. The Fermi Large Area Telescope (LAT) has increased the number of detected gamma-ray MSPs tremendously. Light curve modelling furthermore favours abundant pair production in MSP magnetospheres, so that models of primary cosmic-ray positrons from pulsars should include the contribution from the larger numbers of MSPs and their potentially higher positron output per source. We model the contribution of Galactic MSPs to the terrestrial cosmic-ray electron / positron flux by using a population synthesis code to predict the source properties of present-day MSPs. We simulate pair spectra assuming an offset-dipole magnetic field which boosts pair creation rates. We also consider positrons and electrons that have additionally been accelerated to very high energies in the strong intrabinary shocks in black widow (BW) and redback (RB) binary systems. We transport these particles to Earth by calculating their diffusion and the radiative energy losses they suffer in the Galaxy using a model. Our model particle flux increases for non-zero offsets of the magnetic polar caps. We find that pair cascades from MSP magnetospheres contribute only modestly around a few tens of GeV to the measured fluxes. BW and RB fluxes may reach a few tens of percent of the observed flux up to a few TeV. Future observations should constrain the source properties in this case.