Shock-Cloud Interaction in the Southwestern Rim of RX J1713.7$-$3946 Evidenced by Chandra X-ray Observations.

We report on results of Chandra X-ray observations of the southwestern part of the supernova remnant (SNR) RX~J1713.7$-$3946. We measure proper motions of two X-ray bright blobs, named Blobs A and B, in regions presumably corresponding to the forward shock of the SNR. The measured velocities are $3800 \pm 100~\mathrm{km}~\mathrm{s}^{-1}$ and $2300 \pm 200~\mathrm{km}~\mathrm{s}^{-1}$ for Blobs A and B, respectively. Since a dense molecular clump is located close to Blob B, its slower velocity is attributed to shock deceleration as a result of a shock--cloud interaction. This result provides solid evidence that the forward shock of RX~J1713.7$-$3946 is indeed colliding with dense gas discovered through radio observations reported in literature. The locations and velocity differences of the two blobs lead to an estimate that the shock encountered with the dense gas $\sim 100~\mathrm{yr}$ ago. The shock velocities, together with cutoff energies of the synchrotron X-ray spectra of the blobs, indicate that particle acceleration in these regions is close to the Bohm limit. Blob B, in particular, is almost at the limit, accelerating particles at the fastest possible rate. We discuss possible influence of the shock-cloud interaction on the efficiency of particle acceleration.