ROSAT/ASCAObservations of the Mixed‐Morphology Supernova Remnant W28

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 acentrally-concentrated interior emission, sharply peaked at the center. TheASCA spectra reveal emission lines of Ne, Mg, Si, and Fe K$\alpha$ andcontinuum extending at least up to 7 keV, showing thermal origin with a hotthermal component. We found that spectral variations are present in W28. Thesouthwestern shell can be fit well by a plane-shock model with a temperature of1.5 keV, and the northeastern shell, with a lower temperature of 0.56 keV.Unlike for the southwestern and northeastern shells, the central emissionrequires a two-temperature components with 0.6 keV and 1.8 keV. The lowtemperature 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 isonly ~20 - 25 solar mass. A comparison of W28 with other typicalMixed-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 radiativeshell interacting with clumpy molecular clouds. X-ray emission at its center isa ``fossil'' radiation from gas which was shocked early in the evolution of theremnant, and its centrally-peaked morphology could have been caused byprocesses such as evaporation, electron thermal conduction, and mixing inducedby various hydrodynamical instabilities. But W28 poses a challenge for existingmodels of X-ray emission, because the evaporation model of White & Long (1991)is in conflict with observations, while the presence of temperature variationsseems inconsistent with SNR models with efficient thermal conduction.Comment: total 44 pages including 17 figures. Accepted by Ap