Energy Dependent Morphology in the PWN candidate HESS J1825-137

TL;DR

This paper analyzes the VHE gamma-ray source HESS J1825-137, revealing a spectral steepening with distance from the pulsar, explained by electron cooling, and suggesting it as a prototype for asymmetric pulsar wind nebulae.

Contribution

It presents the first detailed spectral and morphological analysis showing energy-dependent morphology in a PWN, linking gamma-ray and X-ray emission sizes through cooling processes.

Findings

Spectral steepening observed with increasing distance from pulsar

Size discrepancy between gamma-ray and X-ray emissions explained by cooling timescales

HESS J1825-137 serves as a prototype for asymmetric PWN

Abstract

Observations with H.E.S.S. revealed a new source of very high-energy (VHE) gamma-rays above 100 GeV - HESS J1825-137 - extending mainly to the south of the energetic pulsar PSR B1823-13. A detailed spectral and morphological analysis of HESS J1825-137 reveals for the first time in VHE gamma-ray astronomy a steepening of the energy spectrum with increasing distance from the pulsar. This behaviour can be understood by invoking radiative cooling of the IC-Compton gamma-ray emitting electrons during their propagation. In this scenario the vastly different sizes between the VHE gamma-ray emitting region and the X-ray PWN associated with PSR B1823-13 can be naturally explained by different cooling timescales for the radiating electron populations. If this scenario is correct, HESS J1825-137 can serve as a prototype for a whole class of asymmetric PWN in which the X-rays are extended over a much smaller angular scales than the gamma-rays and can help understanding recent detections of X-ray PWN in systems such as HESS J1640-465 and HESS J1813-178. The future GLAST satellite will probe lower electron energies shedding further light on cooling and diffusion processes in this source.