# Detection of a $\gamma$-ray halo around Geminga with the Fermi-LAT and implications for the positron flux

## TL;DR

This study detects a gamma-ray halo around Geminga pulsar using nearly 10 years of Fermi-LAT data, supporting the idea that pulsars contribute significantly to the observed positron excess in cosmic rays.

## Contribution

First analysis combining Fermi-LAT data with HAWC observations to characterize the Geminga gamma-ray halo and estimate its contribution to positron flux.

## Key findings

- Detected gamma-ray halo around Geminga with high significance.
- Estimated diffusion coefficient around 1.1 x 10^{27} cm^2/s at 100 GeV.
- Geminga contributes up to 20% of the positron flux at high energies.

## Abstract

The HAWC Collaboration has discovered a $\gamma$-ray emission extended about 2 degrees around the Geminga and Monogem pulsar wind nebulae (PWNe) at $\gamma$-ray energies $E_\gamma >5$ TeV. We analyze, for the first time, almost 10 years of $\gamma$-ray data obtained with the Fermi Large Area Telescope at $E_\gamma >$ 8 GeV in the direction of Geminga and Monogem. Since these two pulsars are close the Galactic plane we run our analysis with 10 different interstellar emission models (IEMs) to study the systematics due to the modeling of this component. We detect a $\gamma$-ray halo around Geminga with a significance in the range $7.8-11.8\sigma$ depending on the IEM considered. This measurement is compatible with $e^+$ and $e^-$ emitted by the PWN, which inverse-Compton scatter (ICS) with photon fields located within a distance of about 100 pc from the pulsar, where the diffusion coefficient is estimated to be around $1.1 \times 10^{27}$ cm$^2$/s at 100 GeV. We include in our analysis the proper motion of the Geminga pulsar which is relevant for $\gamma$ rays produced for ICS in the Fermi-LAT energy range. We find that an efficiency of about $1\%$ for the conversion of the spin-down energy of the pulsar into $e^+$ and $e^-$ is required to be consistent with $\gamma$-ray data from Fermi-LAT and HAWC. The inferred contribution of Geminga to the $e^+$ flux is at most $20\%$ at the highest energy AMS-02 data. Our results are compatible with the interpretation that the cumulative emission from Galactic pulsars explains the positron excess.