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) γ-ray source HESS J1813–178 with the aim of understanding the origin of the γ-ray emission. Methods. High-angular resolution X-ray studies of the VHE γ-ray emission region are performed using 18.6 ks of XMM-Newton data, taken on HESS J1813–178 in October 2005. Using this dataset we are able to undertake spectral and morphological studies of the X-ray emission object with greater precision than previous studies. NANTEN 12CO(J=1–0) data are used to search for correlations of the γ-ray emission with molecular clouds which could act as target material for γ-ray production in a hadronic scenario. Results. The NANTEN 12CO(J=1–0) observations show a giant molecular cloud of mass 2.5×105 M⊙ at a distance of 4 kpc in the vicinity of HESS J1813–178. Even though there is no direct positional coincidence, this giant cloud might have influenced the evolution of the γ-ray source and its surroundings. The X-ray data show a highly absorbed (nH ∼ 1. × 1023 cm−2) non-thermal X-ray emitting object coincident with the previously known ASCA source AX J1813–178 showing a compact core and an extended tail towards the north-east, located in the centre of the radio shell-type Supernova remnant (SNR) G12.82–0.2. 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. Nevertheless, we cannot distinguish between November 2006 Submitted to Astron.AStrophys SLAC-PUB-12209 2 S. Funk et al.: XMM-Newton and NANTEN observations of HESS J1813–178 the scenarios in which the γ-rays originate in the shell of the SNR and the one in which they originate in the central object. We discuss both scenarios in terms of a one-zone leptonic model and demonstrate, that in order to connect the core X-ray emission to the VHE γ-ray emission electrons have to be accelerated to energies of at least 1 PeV. We conclude that if indeed the X-rays are connected to the VHE γ-rays HESS J1813–178 has to be a Galactic Pevatron.