Centrally Peaked X-Ray Supernova Remnants in the Small Magellanic Cloud Studied with ASCA and ROSAT

Abstract

This paper presents ASCA/SIS and ROSAT/HRI results of three supernova remnants (SNRs) in the Small Magellanic Cloud: 0103-726, 0045-734, and 0057-7226. The ROSAT/HRI images of these SNRs indicate that the most of the X-ray emissions are concentrated in the center region. Only from 0103-726 are faint X-rays along the radio shell also detected. The ASCA/SIS spectra of 0103-726 and 0045-734 exhibit strong emission lines from highly ionized metals. The spectra were well-fitted with non-equilibrium ionization (NEI) plasma models. The metal abundances are found to be larger than the mean chemical compositions in the interstellar medium (ISM) of the SMC. Thus, X-rays from these two SNRs are attributable to the ejecta gas, although the ages estimated from the ionization timescale are significantly large, ~> 10^4 yr. The chemical compositions are roughly consistent with the type-II supernova origin of a progenitor mass ~< 20 M_solar. The SIS spectrum of 0057-7226 was also fitted with an NEI model of an estimated age ~> 6 x 10^3 yr. Although no constraint on the metal abundances was obtained, the rather weak emission lines are consistent with the low metal abundances in the ISM of the SMC. A possible scenario for the evolution of the morphologies and spectra of SNRs is proposed.