MHD simulations of inward shocks in Cassiopeia A

Cassiopeia A, the brightest radio supernova remnant (SNR) in the sky, has several unique characteristics in comparison to its peers. Besides its radio brightness and prominent soft-concave radio spectrum, its gamma-ray spectrum appears to have a low-energy cutoff near 2 GeV, and it is the only SNR with prominent hard X-ray emission. While the unusual radio properties may be attributed to strong emission from reverse shocks, the hard X-ray emission has been associated with high-speed inward shocks induced by high density gases. Then, the low-energy gamma-ray spectral cutoff could be attributed to slow penetration of lower energy particles accelerated near the inward shocks into high-density emission zone. In this paper, we carry out magneto-hydrodynamic (MHD) simulations of shocks in Cassiopeia A and demonstrate that its inward shock structure can indeed be reproduced via shock interactions with clumps of gases with a density of similar to 20 cm(-3).