AN XMM-NEWTON STUDY OF THE MIXED-MORPHOLOGY SUPERNOVA REMNANT W28 (G6.4-0.1)

We have performed an XMM-Newton imaging and spectroscopic study of supernova remnant (SNR) W28, a prototype mixed-morphology or thermal composite SNR believed to be interacting with a molecular cloud. The observed hot X-ray emitting plasma is characterized by low metal abundances, showing no evidence of ejecta. The X-rays arising from the deformed northeastern shell consist of a thermal component with a temperature of similar to 0.3 keV plus a hard component of either thermal (temperature similar to 0.6 keV) or non-thermal (photon index = 0.9-2.4) origin. The X-ray emission in the SNR interior is blobby and the corresponding spectra are best described as the emission from a cold (similar to 0.4 keV) plasma in non-equilibrium ionization with an ionization timescale of similar to 4.3 x 10(11) cm(-3) s plus a hot (similar to 0.8 keV) gas in collisional ionization equilibrium. Applying the two-temperature model to the smaller central regions, we find non-uniform interstellar absorption, temperature, and density distribution, which indicates that the remnant is evolving in a non-uniform environment with denser material in the east and north. The cloudlet evaporation mechanism can essentially explain the properties of the X-ray emission in the center, and thermal conduction may also play a role for length scales comparable to the remnant radius. A recombining plasma model with an electron temperature of similar to 0.6 keV is also feasible for describing the hot central gas with the recombination age of the gas estimated at similar to 2.9 x 10(4) yr.