Constraining the Milky Way's Pulsar Population with the Cosmic-Ray Positron Fraction

TL;DR

This study uses cosmic-ray positron data to constrain local pulsar properties, revealing their significant role in high-energy positron flux and providing insights into pulsar emission efficiencies and spindown timescales.

Contribution

The paper introduces a model linking pulsar characteristics with observed positron fractions, offering new constraints on pulsar emission and spindown parameters based on AMS-02 data.

Findings

Pulsars transfer about 5-20% of their spindown power into high-energy electron-positron pairs.

The positron spectrum is dominated by a few pulsars above 300 GeV.

Best models suggest radio and gamma-ray beams are detectable to 28% and 62% of observers, respectively.

Abstract

Observations of the TeV halos associated with nearby pulsars indicate that these objects inject significant fluxes of very high-energy electron-positrons pairs into the interstellar medium (ISM), thereby likely providing the dominant contribution to the cosmic-ray positron flux. In this paper, we use the cosmic-ray positron fraction as measured by the AMS-02 Collaboration to constrain the characteristics of the local pulsar population. For reasonable model parameters, we find that we can obtain good agreement with the measured positron fraction up to energies of $E_e \sim 300 \, {\rm GeV}$. At higher energies, the positron fraction is dominated by a small number of pulsars, making it difficult to reliably predict the shape of the expected positron fraction. The low-energy positron spectrum supports the conclusion that pulsars typically transfer approximately $\eta \sim 5-20\%$ of their total spindown power into the production of very high-energy electron-positron pairs, producing a spectrum of such particles with a hard spectral index, $\alpha \sim 1.5-1.7$. Such pulsars typically spindown on a timescale on the order of $\tau \sim 10^4 \, {\rm years}$. Our best fits were obtained for models in which the radio and gamma-ray beams from pulsars are detectable to 28% and 62% of surrounding observers, respectively.