Multi-Zone Modeling of Nonthermal Radiation from Pulsar Wind Nebulae

Many pulsar wind nebulae (PWN) are spatially resolved in the radio, X-ray, and even very high energy (VHE) wavebands, and thereby provide an excellent laboratory to study not only pulsar winds and dynamics, but also shock processes, the ambient medium, magnetic feld evolution, and particle transport. Single-zone spectral energy distribution (SED) models have long been used to study the global properties of PWN, but to fully take advantage of high resolution data one must move beyond these simple models. I describe 3-D time-dependent PWN spectral energy distribution modeling, with particular emphasis on the spatial variations within the large and bright PWN associated with PSR J1826-1334, HESS J1825-137. Within this PWN, 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. The large size and high nebular energy budget imply a relatively rapid initial pulsar spin period of 13±7 ms and an age of 40±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) r^(-0.5). The evolution of the cooling break requires an evolving magnetic field which also decreases radially from the pulsar, B(r,t) r^(-0.7)Edot(t)^(0.5). Detection of 10 TeV flux 80 pc from the pulsar requires rapid diffusion of high energy particles with κesc≈90(R/10 pc)²(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±0.1 degree. The growing number of sources with spatially resolved X-ray and VHE measurements (e.g. Vela-X, HESS J1303-631) are prime targets for such multi-zone modeling.