Interpretation of the puzzling gamma-ray spectrum of the Geminga halo

TL;DR

This paper introduces a two-population electron injection model to explain the complex gamma-ray spectrum of the Geminga halo, improving fit over simple models and predicting testable differences in gamma-ray profiles.

Contribution

The paper proposes a novel two-population electron injection model based on X-ray observations, addressing limitations of previous simple models in explaining the Geminga gamma-ray spectrum.

Findings

Two-population model fits HAWC and HESS data well

Predicts different gamma-ray profile energy dependence

Requires a diffusion index slightly larger than 1

Abstract

The gamma-ray halo around Geminga is formed owing to the slow diffusion of the electrons released by the Geminga PWN. The latest HAWC and HESS observations exhibit complex features in the TeV gamma-ray spectrum of the Geminga halo. We first show that the new results cannot be interpreted by the commonly used simple model, where a single power-law injection spectrum and an energy index of $\delta=1/3$ for the diffusion coefficient are assumed. We then propose a two-population electron injection model based on the x-ray observations of the Geminga PWN, which consists of a population of freshly accelerated electrons escaping from the PWN through rapid outflows and a population trapped longer inside the PWN before escaping. The two-population model interprets the HAWC and HESS data well, and the goodness of fit improves significantly compared with the single power-law injection model. It also predicts a different energy dependency of the gamma-ray profile from the single power-law model, which could be tested by LHAASO in the coming future. We note that a $\delta$ slightly larger than 1 is needed to fit the HAWC and HESS data consistently. We also discuss the possible improvements by adopting the two-zone diffusion model.