XMM-Newton observations of HESS J1813-178 reveal a composite Supernova remnant

Aims. We present X-ray and (CO)-C-12(J = 1-0) observations of the very-high-energy (VHE) gamma-ray source HESS J1813-178 with the aim of understanding the origin of the gamma-ray emission. Methods. High-angular resolution X-ray studies of the VHE gamma-ray emission region are performed using 18.6 ks of XMM-Newton data, taken on HESS J1813-178 in October 2005. Using this data set we are able to undertake spectral and morphological studies of the X-ray emission from this object with greater precision than previous studies. NANTEN (CO)-C-12(J=1-0) data are used to search for correlations of the X-ray emission with molecular clouds which could act as target material for X-ray production in a hadronic scenario. Results. The NANTEN (CO)-C-12(J = 1- 0) observations show a giant molecular cloud of mass 2.5 x 10(5) M-circle dot at a distance of 4 kpc in the vicinity of HESS J1813- 178. Even though there is no direct positional coincidence, this giant cloud may have influenced the evolution of the X-ray source and its surroundings. The X- ray data show a highly absorbed ( n(H) similar to 1. x 10(23) cm(-2)) non-thermal X-ray emitting object coincident with the previously known ASCA source AXJ1813-178 exhibiting a compact core and an extended tail towards the north- east, located in the centre of the radio shell- type Supernova remnant ( SNR) G12.82 - 0.02. This central object shows morphological and spectral resemblance to a Pulsar Wind Nebula ( PWN) and we therefore consider that this object is very likely to be a composite SNR. We discuss the scenario in which the gamma-rays originate in the shell of the SNR, and that in which they originate in the central object, in terms of a time- dependent one- zone leptonic model. We demonstrate, that in order to connect the core X-ray emission to the VHE gamma-ray emission electrons have to be accelerated to energies of at least 1 PeV.