On the anomalously large extension of the Pulsar Wind Nebula HESS J1825-137

Understand

The very high energy (VHE) gamma-ray emission reported from a number of pulsar wind nebulae (PWNe) is naturally explained by the inverse Compton scattering of multi-TeV electrons.

• However, the physical dimensions of some gamma-ray-emitting PWNe significantly exceed the scales anticipated by the standard hydrodynamical paradigm of PWN formation.
• The most "disturbing" case in this regard is HESS J1825-137, which extends to distances $r\approx70\rm\,pc$ from the central pulsar PSR J1826-1334.
• If the gamma-ray emission is indeed produced inside the PWN, but not by electrons that escaped the nebula and diffuse in the interstellar medium (ISM), the formation of such an anomalously extended plerion could be realized, in a diluted environment with the hydrogen number density $n_{ism}\le10^{-2}\rm\,cm^{-3}$.