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

Aims.We present X-ray and 12CO(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 12CO(J=1-0) data are used to search for correlations of the $\gamma$-ray emission with molecular clouds which could act as target material for $\gamma$-ray production in a hadronic scenario. Results.The NANTEN 12CO(J=1-0) observations show a giant molecular cloud of mass $2.5 \times 10^5~M_{\odot}$ 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 $\gamma$-ray source and its surroundings. The X-ray data show a highly absorbed ( $n_{{\rm H}} \sim 1. \times 10^{23}$ cm-2) non-thermal X-ray emitting object coincident with the previously known ASCA source AX J1813-178 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.