A possible explanation of the TeV emission from the pulsar wind nebula HESS J1825-137

Pulsar wind nebula (PWN) HESS J1825-137 is one of the largest and brightest gamma-ray sources in Milky Way. It was found that the morphology of the nebula strongly depends on energy and the photon spectrum varies obviously with distance from the pulsar in the >0.1 TeV energy band. In the frame of a spatially-dependent particle transport model, we investigate the TeV gamma-ray observed features of HESS J1825-137 by using the Markov chain Monte Carlo method. Our results are as follows: (i) the observed features can be reasonably explained by the competition between the transport (advection and diffusion) and energy cooling losses (adiabatic, synchrotron, and inverse Compton) of the high energy electrons, and that a parameter ξ ≡ τp/τC is introduced to describe this competition, which is the function of electron energy and position within the nebula, where τp and τC are total transport and energy cooling timescales, respectively; (ii) for the high energy electrons (Ee > 0.1 TeV) within the nebula, the particle diffusion dominates over its advection, and the radiative cooling mostly dominates over adiabatic cooling; and (iii) the change of the radial extent with photon energy Eγ can be expressed as $R\propto E^{-\beta }_\gamma$, where the index β is a function of Eγ and the Klein−Nishina effect has a very important role for the variation of β with Eγ. From the modelling results, the initial rotation period, braking index, and age of the pulsar are predicted to be 12.16 ms, 1.79, and 44 kyr, respectively.