Derivation of the injection spectrum of positrons and electrons from Geminga and Monogem

TL;DR

This study models gamma-ray halos around Geminga and Monogem pulsars to understand particle diffusion and their contribution to local cosmic-ray positrons, revealing suppressed diffusion regions and limited impact on observed positron flux.

Contribution

It provides a unified framework for analyzing pulsar halos, constrains particle transport properties, and estimates their limited role in local positron flux based on gamma-ray data.

Findings

Observed spectra can be reproduced with suppressed local diffusion.

Hard injection spectra with cutoff energies around 10^5–10^6 GeV are favored.

The contribution of these pulsars to local positron flux is limited due to slow diffusion.

Abstract

Extended $\gamma$-ray emission has been observed around several nearby pulsars and is commonly interpreted as inverse-Compton radiation produced by relativistic electrons and positrons diffusing in the surrounding interstellar medium. In this work, a unified analysis of the halos associated with the Geminga and Monogem pulsars is presented, combining GeV--TeV $\gamma$-ray observations within a common physical framework. Assuming continuous injection of $e^\pm$ pairs from the pulsar wind nebulae, the resulting $\gamma$-ray emission is modeled by accounting for particle diffusion and radiative energy losses. I find that the observed spectra of both Geminga and Monogem can be reproduced within this framework, provided that particle transport in the vicinity of the sources is significantly suppressed with respect to the average Galactic diffusion. The fits favor hard injection spectra and cutoff energies of order $10^5$--$10^6$~GeV, consistent with efficient lepton acceleration in pulsar environments. Using the best-fit injection models inferred from the $\gamma$-ray data, then I estimate the contribution of Geminga and Monogem to the local cosmic-ray positron flux measured by AMS-02. I find that the slow-diffusion region surrounding the sources strongly suppresses the positron flux reaching the Earth, leading to a subdominant contribution over most of the AMS-02 energy range, with a possible effect only near the upper end of the measured spectrum. The results support an interpretation in which TeV halos trace regions of inhibited particle diffusion around pulsars, while at the same time implying only a limited impact on the local positron flux. This combined analysis highlights the importance of extended $\gamma$-ray observations for constraining particle transport in the vicinity of Galactic cosmic-ray sources.