Numerically modelling the Cygnus Loop as a remnant evolved in an anisotropic cavity.

The morphology of the middle-aged supernova remnant, Cygnus Loop, seen in X-rays, is peculiar, with a blowout in the south region and other irregular features, such as a bump in the west, a limb with a planar morphology in the east and asymmetry between the east and the west shock profiles of the blowout. The detailed process of the formation of the peculiar profile of the shock is still unclear. We perform three-dimensional hydrodynamical simulations for the remnant to revisit its evolution. In the simulations, the progenitor ejects an anisotropic, latitude-dependent wind, and travels in a direction that is not aligned with the symmetry axis of the wind. As a result, a cavity with a fringed structure is produced. The remnant has evolved in the cavity for about 104 yr. In the north-east, the shock has first encountered the bow shock, and this part corresponds to the bright north-eastern region. The south blowout is formed due to the shock travelling into the undisturbed wind, and the interaction of the shock with the cavity leads to the other peculiar features of the shock structure. The resulting profile of the remnant is consistent with that indicated in X-rays. It can be concluded that the supernova explosion occurred in the cavity produced by an anisotropic stellar wind experiencing two main phases with different wind velocities. [ABSTRACT FROM AUTHOR]