Toward Understanding the Cosmic-ray Acceleration at Young Supernova Remnants Interacting with Interstellar Clouds: Possible Applications to RX J1713.7-3946

Abstract: Using three-dimensional magnetohydrodynamics simulations, we investigate general properties of a blast wave shock interacting with interstellar clouds. The shock-cloud interaction generates a turbulent shell through the vorticity generations. In the turbulent shell, the magnetic field is amplified as a result of turbulent dynamo action. In the case of a young supernova remnant, the corresponding strength of the magnetic field is approximately 1 mG. The propagation speed of the shock wave is significantly stalled in the clouds because of the high density, while the shock maintains a high velocity in the diffuse surrounding. In addition, when the shock wave hits the clouds, reflection shock waves are generated that propagate back into the shocked shell. From these simulation results, many observational characteristics of a young SNR RX J1713.7-3946 that is suggested to be interacting with molecular clouds, can be explained as follows: The reflection shocks can accelerate particles in the turbulent downstream region where the magnetic field strength reaches 1mG, which causes short-time variability of synchrotron X-rays. Since the shock velocity is stalled locally in the clouds, the temperature in the shocked cloud is suppressed below 1 keV. Thus, thermal X-ray line emissions would be faint even if the SNR is interacting with molecular clouds. We also find that the photon index of the $\pi^0$-decay gamma rays generated by cosmic-ray protons can be 1.5, because the penetration depth of high-energy particles into the clumpy clouds depends on their energy, which is consistent with the recent gamma-ray observation by Fermi space telescope.