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 .