Slow diffusion is necessary to explain the gamma-ray pulsar halos

TL;DR

This paper investigates whether fast diffusion or ballistic transport can explain gamma-ray halos around pulsars, concluding that slow diffusion is essential for accurate modeling of these halos.

Contribution

It demonstrates that while ballistic transport can explain some halos, slow diffusion remains necessary for a comprehensive understanding of pulsar gamma-ray halos.

Findings

Ballistic regime can marginally fit Geminga halo in fast diffusion scenario.

LHAASO halo around PSR J0622+3749 cannot be explained without slow diffusion.

High conversion efficiency (>100%) is required if ballistic transport is assumed.

Abstract

It was suggested that the $\gamma$-ray halo around Geminga might not be interpreted by slow-diffusion. If the ballistic regime of electron/positron propagation is considered, the Geminga halo may be explained even with a large diffusion coefficient. In this work, we examine this effect by taking the generalized J\"uttner propagator as the approximate relativistic Green's function for diffusion and find that the morphology of the Geminga halo can be marginally fitted in the fast-diffusion scenario. However, the recently discovered $\gamma$-ray halo around PSR J0622$+$3749 at LHAASO cannot be explained by the same effect and slow diffusion is the only solution. Furthermore, both the two pulsar halos require a conversion efficiency from the pulsar spin-down energy to the high energy electrons/positrons much larger than 100\%, if they are interpreted by this ballistic transport effect. Therefore, we conclude that slow diffusion is necessary to account for the $\gamma$-ray halos around pulsars.