X-Ray Observations of the Supernova Remnant W28 with Suzaku --- I.   Spectral Study of the Recombining Plasma
  We present the Suzaku results of the mixed-morphology supernova remnant W28. The X-ray spectra of the central region of W28 exhibit many bright emission lines from highly ionized atoms. An optically thin thermal plasma in collisional ionization equilibrium, either of single-temperature or multi-temperature failed to reproduce the data with line-like and bump-like residuals at the Si Lyman$\alpha$ energy and at 2.4--5.0 keV, respectively. The bumps probably correspond to radiative recombination continua from He-like Si and S. A simple recombining plasma model nicely fit the bump structures, but failed to fit low energy bands. The overall spectra can be fit with a multi-ionization temperature plasma with a common electron temperature. The multi-ionization temperatures are interpreted as elemental difference of ionization and recombination timescales. These results prefer the rarefaction scenario for the origin of the recombining plasma. 
Makoto Sawada and Katsuji Koyama
astro-ph.HE