X-ray emissions from the Cygnus Loop supernova remnant are studied with the Japanese X-ray astronomy satellite, ASCA. The energy resolving power of the X-ray CCD camera, SIS, onboard ASCA enables us to resolve the individual emission lines from the continuum emission between 0.5-10 keV energy range where there are many emission lines from O to Fe.
The Cygnus Loop is one of the best studied and brightest SNRs in various wavelengths. Its large apparent size, high surface brightness, and low absorption features have made the Cygnus Loop to be an ideal target for the study of the spatially-resolved spectroscopic structure in detail. The apparent size (84" in radius) is too large to cover with a single pointing observation with the SIS (FOV of 22" square). We have observed the Cygnus Loop with more than 10 different pointings.
We find the significant spectral variations from various pointing regions. Applying the non equilibrium ionization (NEI) model coded by Masai (1984), we confirm the NEI condition at all positions. The obtained kTe structure radially increases toward the center with some local maxima between shell regions and the center of the Loop. Abundances of heavy elements are lower than those of thecosmic values, whereas higher abundances are obtained at the Center, the western arm of the V-shaped region, and the Southern regions. We, next, investigate the spatial variations of kTe, ionization timescales, and abundances of heavy elements. Apparently, the abundances of O, Ne, Mg, Si, and Fe increases toward the center.
There are mainly two results we find. One is the clarification of the heating mechanism at regions with complicated surface brightness; the Northern-region and the Eastern region. In both region, the reflection shock heating can explain the whole X-ray properties like the temperature variation as well as the surface brightness.
The other result we find is a lot of signatures of the ejecta at supernova explosion. Since the Cygnus Loop is a fairly evolved remnant, it has been considered that the X-ray emitting plasma is dominated by the interstellar matter swept up by the supernova blast wave. The high energy resolution spectra obtained with the SIS reveal that the ejecta are mainly distributed inside of 0.9 Rs, where Rs is the shock radius. X-ray emissions both from the Center and from the Southern region are dominated not by the swept up ISM but by the ejecta. At the V-shaped region in the south-west part of the Cygnus Loop, we find that the X-ray spectra from two arms of the V-shaped region are different in abundance. The swept up ISM is dominated at the western arm whereas the eastern arm is constituted by the ejecta fragment surviving from the explosion.
The composition of heavy elements at various portions of the Cygnus Loop accept both SN type Ia and II (or Ib) model. The fragmentation of the ejecta and relatively low abundance of Fe strongly suggest the massive progenitor in origin of the Cygnus Loop.