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.

This article focuses on a reanalysis of the Suzaku X-ray spectrum of the Galactic supernova remnant (SNR) W 49B to investigate the origin of its recombining plasma and the presence of charge-exchange (CX) emission. By comparing spectral fits using different plasma codes—AtomDB and SPEX—and accounting for systematic uncertainties including instrumental gain calibration, the study finds that the previously reported Fe xxv CX emission at 8–9 keV is likely an artifact caused by missing high-shell dielectronic recombination satellite lines in earlier spectral models. Revised measurements of Fe-group ejecta mass ratios (Mn/Cr and Ni/Fe) show better agreement with theoretical nucleosynthesis models for both Type Ia and spherical core-collapse supernova explosions, though uncertainties remain. The findings challenge the conduction cooling scenario as the dominant mechanism for the recombining plasma in W 49B, given the lack of robust CX emission evidence, and suggest that rapid adiabatic cooling (rarefaction) in a dense circumstellar medium remains a plausible explanation. The study underscores the critical importance of accurate spectral modeling for interpreting X-ray CCD data and highlights the need for future high-resolution spectroscopy with missions like XRISM to resolve remaining ambiguities.