Aims . We present a detailed view of the pulsar wind nebula ( PWN ) HESS J1825–137 .
We aim to constrain the mechanisms dominating the particle transport within the nebula , accounting for its anomalously large size and spectral characteristics . Methods . The nebula was studied using a deep exposure from over 12 years of H.E.S.S .
I operation , together with data from H.E.S.S .
II that improve the low-energy sensitivity .
Enhanced energy-dependent morphological and spatially resolved spectral analyses probe the very high energy ( VHE , E > 0.1 TeV ) \gamma -ray properties of the nebula . Results . The nebula emission is revealed to extend out to 1.5 ^ { \circ } from the pulsar , \sim 1.5 times farther than previously seen , making HESS J1825–137 , with an intrinsic diameter of \sim 100 pc , potentially the largest \gamma -ray PWN currently known .
Characterising the strongly energy-dependent morphology of the nebula enables us to constrain the particle transport mechanisms .
A dependence of the nebula extent with energy of R \propto E ^ { \alpha } with \alpha = -0.29 \pm 0.04 _ { \mathrm { stat } } \pm 0.05 _ { \mathrm { sys } } disfavours a pure diffusion scenario for particle transport within the nebula .
The total \gamma -ray flux of the nebula above 1 TeV is found to be ( 1.12 \pm 0.03 _ { \mathrm { stat } } \pm 0.25 _ { \mathrm { sys } } ) \times 10 ^ { -11 } \mathrm { cm } ^ { -2 } \mathrm { s } ^ { -1 } , corresponding to \sim 64 \% of the flux of the Crab nebula . Conclusions . HESS J1825-137 is a PWN with clearly energy-dependent morphology at VHE \gamma -ray energies .
This source is used as a laboratory to investigate particle transport within intermediate-age PWNe .
Based on deep observations of this highly spatially extended PWN , we produce a spectral map of the region that provides insights into the spectral variation within the nebula .