Multi-Zone Modeling of The Pulsar Wind Nebula HESS J1825-137

TL;DR

This study models the spatial and spectral properties of the pulsar wind nebula HESS J1825-137 across multiple wavelengths, revealing insights into electron transport, magnetic field evolution, and particle diffusion processes.

Contribution

It presents a comprehensive 3-D time-dependent spectral energy distribution model incorporating spatial variations, magnetic field evolution, and rapid particle diffusion, advancing understanding of pulsar wind nebula dynamics.

Findings

Electron cooling causes spectral softening with distance from the pulsar.

Radial magnetic field decrease influences nebula's emission properties.

Rapid diffusion of high-energy particles explains distant TeV flux detection.

Abstract

The pulsar wind nebula associated with PSR J1826-1334, HESS J1825-137, is a bright very high energy source with an angular extent of ~1 degree and spatially-resolved spectroscopic TeV measurements. The gamma-ray spectral index is observed to soften with increasing distance from the pulsar, likely the result of cooling losses as electrons traverse the nebula. We describe analysis of X-ray data of the extended nebula, as well as 3-D time-dependent spectral energy distribution modeling, with emphasis on the spatial variations within HESS J1825-137. The multi-wavelength data places significant constraints on electron injection, transport, and cooling within the nebula. The large size and high nebular energy budget imply a relatively rapid initial pulsar spin period of 13 \pm 7 ms and an age of 40 \pm 9 kyr. The relative fluxes of each VHE zone can be explained by advective particle transport with a radially decreasing velocity profile with $v(r) \propto r^{-0.5}$. The evolution of the cooling break requires an evolving magnetic field which also decreases radially from the pulsar, $B(r,t) \propto r^{-0.7} \dot{E}(t)^{1/2}$. Detection of 10 TeV flux ~80 pc from the pulsar requires rapid diffusion of high energy particles with $\tau_{esc} \approx 90 (R / 10 pc)^2 (E_e/100 TeV)^{-1}$ year, contrary to the common assumption of toroidal magnetic fields with strong magnetic confinement. The model predicts a rather uniform Fermi LAT surface brightness out to ~1 degree from the pulsar, in good agreement with the recently discovered LAT source centered 0.5 degree southwest of PSR J1826-1334 with extension 0.6 \pm 0.1 degree.