The Origin of Recombining Plasma and the Detection of the Fe-K Line in the Supernova Remnant W28

TL;DR

This study presents spatially resolved X-ray spectroscopy of supernova remnant W28, revealing recombining plasma in different regions and suggesting thermal conduction and cosmic-ray interactions as key processes in its plasma evolution.

Contribution

It provides the first spatially resolved analysis of recombining plasma in W28, highlighting the role of thermal conduction and cosmic-ray ionization in plasma formation.

Findings

Recombining plasma detected in both the center and northeastern rim of W28.

Thermal conduction likely causes the recombining plasma in the northeastern rim.

Fe I Kα line excess suggests cosmic-ray induced inner shell ionization.

Abstract

Overionized recombining plasmas (RPs) have been discovered from a dozen of mixed- morphology (MM) supernova remnants (SNRs). However their formation process is still under debate. As pointed out by many previous studies, spatial variations of plasma temperature and ionization state provide clues to understand the physical origin of RPs. We report on a spatially resolved X-ray spectroscopy of W28, which is one of the largest MM SNRs found in our Galaxy. Two observations with Suzaku XIS cover the center of W28 to the northeastern rim where the shock is interacting with molecular clouds. The X-ray spectra in the inner regions are well reproduced by a combination of two-RP model with different temperatures and ionization states, whereas that in northeastern rim is explained with a single-RP model. Our discovery of the RP in the northeastern rim suggests an effect of thermal conduction between the cloud and hot plasma, which may be the production process of the RP. The X-ray spectrum of the north- eastern rim also shows an excess emission of the Fe I K{\alpha} line. The most probable process to explain the line would be inner shell ionization of Fe in the molecular cloud by cosmic-ray particles accelerated in W28.