We present three sets of ROSAT Position Sensitive Proportional Counter and four sets of 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. There are also partial northeastern and southwestern shells, and both the central and shell X-ray emission is highly patchy. The ASCA spectra reveal emission lines of Ne, Mg, Si, and Fe Kα and continuum extending at least up to 7 keV, showing that X-ray emission in W28 is mostly of thermal origin with a hot thermal component. We found that spectral variations are present in W28. The southwestern shell can be fitted well by a plane-shock model with a temperature of 1.5 keV and an ionization timescale of 1.5 × 1011 cm-3 s. The long ionization timescale combined with a low estimated electron density of ~0.2 cm-3 imply an SNR age of several times 104 yr. The low density in the southwest is consistent with the shock breakout away from molecular clouds in the north and northeast. The northeastern shell, with a lower temperature of 0.56 keV and a longer ionization timescale of 1.7 × 1013 cm-3 s, spatially coincides with the radiative shell delineated by radio and optical filaments, but a relatively high temperature and a low density of X-ray-emitting gas in the northeastern shell indicate that we are not observing gas cooling from high temperatures. Unlike for the southwestern and northeastern shells, the central emission cannot be fitted well by a single-temperature model, but two components with temperatures of 0.6 and 1.8 keV are required. The long ionization timescales imply that the gas is close to the ionization equilibrium. The low-temperature component is similar to those seen in other mixed-morphology SNRs. The X-ray luminosity of W28 is ~6 × 1034 ergs s-1, and the estimated X-ray mass is only ~20-25 M☉. A comparison of W28 with other typical mixed-morphology SNRs reveals significant differences in its X-ray properties; W28 has a higher temperature and noticeable spectral variations. W28 belongs to a class of SNRs considered by Chevalier, with a radiative shell interacting with clumpy molecular clouds. X-ray emission at its center is a "fossil" radiation from gas that 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 is in conflict with observations, while the presence of temperature variations seems inconsistent with SNR models with efficient thermal conduction.
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