Positron flux and gamma-ray emission from Geminga pulsar and pulsar wind nebula

TL;DR

This paper investigates gamma-ray emissions from the Geminga pulsar and nebula, proposing a two-zone diffusion model that explains observed emissions and positron excess, challenging standard cosmic ray propagation assumptions.

Contribution

It introduces a two-zone diffusion model with slow diffusion in the nebula and fast diffusion in the ISM, explaining gamma-ray observations and positron excess simultaneously.

Findings

A two-zone diffusion model fits HAWC data and positron excess.

Diffusion in the nebula is slower than in the ISM.

Implied ISM diffusion coefficient aligns with spiral arm model predictions.

Abstract

Nearby pulsars have been suggested as sources of $\sim$TeV $e^+/e^-$ Cosmic Ray (CR) excess on Earth. The High-Altitude Water Cherenkov Observatory (HAWC) detected extended TeV emission regions in the direction of two nearby middle-aged pulsars, Geminga and PSR B0656+14. By modeling the TeV emission as inverse Compton emission from electron-positron pairs diffusing in the interstellar medium (ISM), the HAWC collaboration derives a diffusion coefficient much smaller than the standard value in the vicinity of the two pulsars, which make them unlikely the origin of the positron excess. We propose that the observed $\gamma$-ray emission originate from the relic pulsar wind nebula. A two zone diffusion model with a slow diffusion in the nebula and a fast diffusion in the ISM can explain the HAWC surface brightness profile and the positron excess simultaneously. Inefficient diffusion in the $\gamma$-ray emission region surrounding a middle-aged pulsar maybe a common phenomenon that can be tested by future observation. The implied diffusion coefficient in the ISM is smaller than the one suggested by the standard CR propagation model, but it is fully consistent with the predictions of the spiral arm model.