The Plasma Structure of the Cygnus Loop from the Northeastern Rim to the Southwestern Rim

The Cygnus Loop was observed from the northeast to the southwest with XMM-Newton. We divided the observed region into two parts, the north and the south path, and studied the X-ray spectra along the two. The spectra can be well fitted either by a one-component nonequilibrium ionization (NEI) model or by a two-component NEI model. The rim regions can be well fitted by a one-component model with relatively low kTe whose metal abundances are subsolar (0.1-0.2 Z☉). The major part of the paths requires a two-component model. Because of projection effects, we conclude that the low-kTe (~0.2 keV) component surrounds the high-kTe (~0.6 keV) component, with the latter having relatively high metal abundances (~5 times solar). Since the Cygnus Loop is thought to have originated in a cavity explosion, the low-kTe component thus originates from the cavity wall, while the high-kTe component originates from the ejecta. The flux of the cavity-wall component shows a large variation along our paths. We find it to be very thin in the southwest region, suggesting a blowout along our line of sight. The metal distribution inside the ejecta exhibits nonuniformity, depending on the element: O, Ne, and Mg are relatively more abundant in the outer region, while Si, S, and Fe are concentrated in the inner region, with all metals showing strong asymmetry. This observational evidence implies an asymmetric explosion of the progenitor star. The abundance of the ejecta also indicates that the progenitor mass was about 15 M☉.