Revisiting the Suzaku spectrum of the Galactic SNR W 49B: Non-detection of iron K-shell charge-exchange emission and refined ejecta-mass ratios of iron-group elements

Abstract

The origin of the recombining plasma in several Galactic SNRs has been debated. A plausible mechanism would be a rapid cooling in the past, by either an adiabatic or a conductive process. A recent spectral study of W 49B reported a possible charge-exchange emission due to collisions between the shock-heated ejecta and cold clouds, which could be direct support for the conduction cooling scenario. However, a potentially large systematic uncertainty in the spectral analysis has not been examined. In this paper, we revisit the Suzaku spectrum of W 49B, taking into account the systematic uncertainties in spectral codes and instrumental gain calibration. We find that the previously reported flux of the charge-exchange emission is fully attributable to high-shell transition lines, mainly of dielectronic recombination satellites, that are missing from the spectral codes. We also report refined Fe-group ejecta-mass ratios, which, in comparison to those in the literature, show better agreement with theoretical expectations from nucleosynthesis models, either of Type Ia explosions or spherical core-collapse explosions.