2012ApJ...746...82F

RX J1713.7-3946 is the most remarkable TeV γ-ray supernova remnant (SNR) that emits γ-rays in the highest energy range. We have made a new combined analysis of CO and H I in the SNR and derived the total protons in the interstellar medium (ISM). We have found that the inclusion of the H I gas provides a significantly better spatial match between the TeV γ-rays and ISM protons than the H₂ gas alone. In particular, the southeastern rim of the γ-ray shell has a counterpart only in the H I. The finding shows that the ISM proton distribution is consistent with the hadronic scenario that cosmic-ray (CR) protons react with ISM protons to produce the γ-rays. This provides another step forward for the hadronic origin of the γ-rays by offering one of the necessary conditions missing in the previous hadronic interpretations. We argue that the highly inhomogeneous distribution of the ISM protons is crucial in the origin of the γ-rays. Most of the neutral gas was likely swept up by the stellar wind of an OB star prior to the supernova (SN) explosion to form a low-density cavity and a swept-up dense wall. The cavity explains the low-density site where the diffusive shock acceleration of charged particles takes place with suppressed thermal X-rays, whereas the CR protons can reach the target protons in the wall to produce the γ-rays. The present finding allows us to estimate the total CR proton energy to be ∼10⁴⁸ erg, 0.1% of the total energy of the SN explosion.