We present three sets of ROSAT PSPC 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 central and shell X-ray emission is highly patchy .

The ASCA spectra reveal emission lines of Ne , Mg , Si , and Fe K \alpha 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 fit well by a plane-shock model with a temperature of 1.5 keV and an ionization timescale of 1.5 \times 10 ^ { 11 } cm ^ { -3 } s. The long ionization timescale combined with a low estimated electron density of \sim 0.2 cm ^ { -3 } implies SNR age of several \times 10 ^ { 4 } 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 \times 10 ^ { 13 } 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 can not be fit well by a single temperature model , but two components with temperatures of 0.6 keV 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 \sim 6 \times 10 ^ { 34 } ergs s ^ { -1 } , and the estimated X-ray mass is only \sim 20 - 25 M _ { \odot } .
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 ( 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 .