X-Ray Observations of the Supernova Remnant W28 with Suzaku --- I. Spectral Study of the Recombining Plasma

TL;DR

This study analyzes Suzaku X-ray data of supernova remnant W28, revealing a recombining plasma state with multi-ionization temperatures, supporting a rarefaction origin scenario.

Contribution

First detailed spectral analysis of W28 revealing recombining plasma with multi-ionization temperatures, advancing understanding of supernova remnant plasma states.

Findings

Recombining plasma identified in W28 with radiative recombination continua.

Multi-ionization temperature plasma model fits the spectra.

Results support the rarefaction scenario for plasma origin.

Abstract

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.