# On the Unusually Large Spatial Extent of the TeV nebula HESS J1825-137: Implication from the Energy-Dependent Morphology

## TL;DR

This study uses deep HESS observations to analyze the energy-dependent shape of the pulsar wind nebula HESS J1825-137, revealing diffusion-dominated particle transport as a key factor in its large spatial extent and offering insights into pulsar history.

## Contribution

It demonstrates that the nebula's large size can be explained by diffusion of electrons/positrons, providing a new interpretation of its morphology and implications for particle transport models.

## Key findings

- Energy-dependent morphology consistent with diffusion-dominated transport
- Large nebula size explained by diffusion of escaping particles
- Morphology can inform pulsar spin-down history

## Abstract

Deep observation of the High Energy Stereoscopic System (HESS) on the most extended pulsar wind nebula HESS J1825-137 reveals an enhanced energy-dependent morphology, providing useful information on the particle transport mechanism in the nebula. We find that the energy-dependent morphology is consistent with a diffusion-dominated transport of electrons/positrons. It provides an alternative possible interpretation for the unusually large spatial extent (i.e., $\gtrsim 100$pc) of the nebula, which could then be attributed to the diffusion of escaping electrons/positrons from a compact plerion. The influence of various model parameters on the energy-dependent extent of the nebula is studied in the diffusion-dominated scenario. We also show that the energy-dependent morphology of the nebula may also be used to study the spin-down history of the pulsar.