Centrally peaked X-ray supernova remnants in the Small Magellanic Cloud studied with ASCA and ROSAT

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, greater than or similar to 10(4)yr. The chemical compositions are roughly consistent with the type-II supernova origin of a progenitor mass less than or similar to 20 M-circle dot. The SIS spectrum of 0057-7226 was also fitted with an NEI model of an estimated age greater than or similar to 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.