Spatial identification of the overionized plasma in W49B

TL;DR

This study spatially maps overionized plasma in supernova remnant W49B, revealing its concentration in the center and western jet, and linking rapid cooling to ejecta expansion dynamics.

Contribution

It provides the first spatial localization of overionized plasma in W49B, clarifying its physical origin and distribution within the remnant.

Findings

Overionized plasma is localized in the center and western jet.

Overionization is absent in the eastern jet due to dense cloud interaction.

Rapid cooling of inner ejecta is driven by expansion, unlike outer ejecta.

Abstract

Recent Suzaku X-ray observations of the ejecta-dominated supernova remnant W49B have shown that in the global spectrum there is a clear indication for the presence of overionized plasma whose physical origin is still under debate. In order to ascertain the physical origin of such a rapidly cooling plasma, we focus on the study of its spatial localization within the X-ray emitting ejecta. We confirm the presence of a saw-edged excess (interpreted as a strong radiative recombination continuum) in the global spectrum above 8 keV, emerging above the ionization-equilibrium model. We produce a hardness ratio map to determine where the plasma is overionized and we perform a spectral analysis of the regions with and without strong overionization. We find that the overionized plasma is localized in the center of the remnant and in its western jet, while it is not detected in the bright eastern jet, where the expansion of the ejecta is hampered by their interaction with a dense interstellar cloud. The location of overionized plasma suggests that the inner ejecta are rapidly cooling by expansion, unlike the outer ejecta, for which expansion is hampered by interstellar clouds seen in H2