Revisiting the Geminga halo at GeV energies with Fermi-LAT data

TL;DR

This study re-analyzed the Geminga pulsar halo using Fermi-LAT data across 1-1000 GeV, finding low significance of halo emission due to background modeling challenges and revealing a very hard spectrum in the 1-10 GeV range.

Contribution

It extends previous analyses by covering a broader energy range with more data, highlighting background modeling issues and providing spectral insights into the Geminga halo.

Findings

Low significance of halo emission in 1-10 GeV range

Background modeling challenges over broad energy range

Detection of a very hard spectrum in 1-10 GeV

Abstract

Nearby pulsars within $\sim1\,{\rm kpc}$ are considered to be possible sources of 10-500 GeV cosmic-ray positron excess measured by PAMELA and AMS-02. A TeV halo around Geminga is detected by HAWC, and the measurements of its surface brightness profile indicate a slow particle diffusion surrounding the source. This result challenges the pulsar interpretation of the positron excess. The observations at GeV energies provide direct information on the electron/positron density in the GeV nebula, which can offer more direct constraints on the origin of the positron excess. Two previous works have performed analyses on the GeV emission of the pulsar halo, but focused on the energy band above 8 GeV. In this work, we use a longer dataset from the Fermi Large Area Telescope (LAT) to re-analyze the GeV halo emission of Geminga, extending the analysis to cover the energy range of 1-1000 GeV. We find that the analysis in this wider energy range results in a low significance of the halo emission. This can be attributed to the Galactic interstellar emission model being unable to perfectly fit the background over this broader energy range, and due to the low measured halo flux at $<$ 10 GeV energies leading to a mismatch between the observation and model expectation. We also derive the spectral energy distribution of the tentative halo emission, which shows a very hard spectrum in the 1-10 GeV range.