Energy dependent γ-ray morphology in the pulsar wind nebula HESS J1825-137

Aims.We present results from deep $\gamma$-ray observations of the Galactic pulsar wind nebula HESS J1825-137 performed with the HESS array. Methods. Detailed morphological and spatially resolved spectral studies reveal the very high-energy (VHE) $\gamma$-ray aspects of this object with unprecedented precision. Results. We confirm previous results obtained in a survey of the Galactic Plane in 2004. The $\gamma$-ray emission extends asymmetrically to the south and south-west of the energetic pulsar PSR J1826-1334 , that is thought to power the pulsar wind nebula. The differential $\gamma$-ray spectrum of the whole emission region is measured over more than two orders of magnitude, from 270 GeV to 35 TeV, and shows indications for a deviation from a pure power law. Spectra have also been determined for spatially separated regions of HESS J1825-137. The photon indices from a power-law fit in the different regions show a softening of the spectrum with increasing distance from the pulsar and therefore an energy dependent morphology. Conclusions. This is the first time that an energy dependent morphology has been detected in the VHE $\gamma$-ray regime. The VHE $\gamma$-ray emission of HESS J1825-137 is phenomenologically discussed in the scenario where the $\gamma$-rays are produced by VHE electrons via Inverse Compton scattering. The high $\gamma$-ray luminosity of the source cannot be explained on the basis of constant spin-down power of the pulsar and requires higher injection power in past.