ROSAT/Asca Observations of the Mixed-Morphology Supernova Remnant W28

Abstract

We present ROSAT PSPC and ASCA observations of the supernova remnant (SNR) W28. The overall shape of X-ray emission in W28 is elliptical, dominated by a centrally-concentrated interior emission, sharply peaked at the center. The ASCA spectra reveal emission lines of Ne, Mg, Si, and Fe K$\alpha$ and continuum extending at least up to 7 keV, showing thermal origin with a hot thermal component. We found that spectral variations are present in W28. The southwestern shell can be fit well by a plane-shock model with a temperature of 1.5 keV, and the northeastern shell, with a lower temperature of 0.56 keV. Unlike for the southwestern and northeastern shells, the central emission requires a two-temperature components with 0.6 keV and 1.8 keV. The low temperature component is similar to those seen in other Mixed-morphology SNRs. The X-ray luminosity of W28 is 6x 10^34 ergs/s, and the estimated X-ray mass is only ~20 - 25 solar mass. A comparison of W28 with other typical Mixed-morphology SNRs reveals significant differences in its X-ray properties; W28 has a significantly higher temperature and noticeable spectral variations. W28 belongs to a class of SNRs considered by Chevalier (1999), with a radiative shell interacting with clumpy molecular clouds. X-ray emission at its center is a ``fossil'' radiation from gas which was shocked early in the evolution of the remnant, and its centrally-peaked morphology could have been caused by processes such as evaporation, electron thermal conduction, and mixing induced by various hydrodynamical instabilities. But W28 poses a challenge for existing models of X-ray emission, because the evaporation model of White & Long (1991) is in conflict with observations, while the presence of temperature variations seems inconsistent with SNR models with efficient thermal conduction.

total 44 pages including 17 figures. Accepted by ApJ

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