Discovery of the Radio and X-ray Counterpart of Tev Γ-ray Source Hess J1731-347

We discover a faint shell-type radio and X-ray source, G353.6-0.7, associated with HESS J1731-347. G353.6-0.7 is likely an old supernova remnant (SNR), based on radio (0.8 GHz, 1.4 GHz and 5 GHz), infrared (8 µm from the GLIMPSE Legacy Project and 21 µm from the Midcourse Space Experiment), and X-ray (0.1 keV-2.4 keV from the ROSAT survey and 5-20 keV from the INTEGRAL survey) data. The SNR, centered at (l, b)=(353.55,-0.65) with a radius of ∼ 0.25 • , closely matches the outline of the recently discovered extended TeV source HESS J1731-347, which has no previously identified counterpart. A diffuse X-ray enhancement detected in the ROSAT all-sky survey is coincident with lower half shell of the SNR. Therefore the SNR is the best radio counterpart of both the HESS source and the diffuse X-ray enhancement. G353.6-0.7 has an age of ∼ 27000 yrs. Altogether, the new discovery provides the best case that an old SNR emits TeV γ-rays. : 1. INTRODUCTION About 52 Galactic TeV γ-ray sources have been detected by ground-based high energy telescopes (e.g. the High Energy Stereoscopic System (HESS), Aharonian et al. 2006; the MAGIC telescopes, Albert et al. 2006) so far. Some of them do not have counterparts in other wavebands. Seeking their Galactic counterparts in other wavebands could play a key role for us to understand the origin of these sources and distinguish different acceleration mechanisms to emit TeV γ-rays. Among identified counterparts, young shell-type supernova remnants (SNRs) are one of the four Galactic populations to generate very high energy (VHE) γ-rays, i.e. pulsar wind nebulae (PWN), X-ray binaries, shell SNRs and a young stellar cloud Westerlund 2 (likely association of an old SNR with a TeV source up to now. We report here the discovery of a new faint radio and X-ray shell which closely matches the extended TeV source HESS J1731-347 without previously identified counterpart (Aharonian et al. 2008). We use new radio, infrared and X-ray data to identify the shell as an old SNR, therefore obtaining second association of a TeV source with an old SNR. The 1420 MHz radio continuum and 21 cm HI emission data sets come from the Southern Galactic Plane Survey (SGPS) which is described in detail by Haverkorn et al. (2006) and McClure-Griffiths et al. (2005). The SGPS images the HI line emission and 1420 MHz continuum in the range 253 • < l < 357 • …