Initial state of the recombining plasma in supernova remnant W 28

We have investigated supernova remnant (SNR) W 28 using Suzaku archival data, and we report here the results of our spatially resolved analyses. We carried out spectral analyses using a recombining-plasma (RP) model with element-dependent initial ionization temperatures, obtaining similar to 0.5 keV for Ne, similar to 0.7 keV for Mg, similar to 1.0 keV for Si, similar to 1.2 keV for S, similar to 1.4 keV for Ar, similar to 1.7 keV for Ca, and similar to 0.7 keV for Fe in the initial RP phase. In addition to the north-east regions where RPs have been reported previously, we find that the ionization temperatures in the south-east and south-west regions exhibit trends that are similar to those in the central region in the initial RP phase. We also find that the time elapsed from the initial phase of the RP to the present is shorter in the central region, similar to 300 yr, and longer in the outside regions, similar to 10(3)-10(4) yr. Our results cannot be explained by simple scenarios involving thermal conduction into molecular clouds or by adiabatic cooling (due to rarefaction), indicating that more complex mechanisms or other scenarios are required. We also estimate the ejecta mass to be ?14 M-?, which indicates that this SNR was produced by a massive star.