Overionization in X-ray spectra: a new paradigm for Mixed-Morphology SNRs

TL;DR

This paper reviews the discovery of over-ionized ejecta in mixed-morphology supernova remnants, challenging existing models and proposing new explanations for their unique X-ray spectral features.

Contribution

It introduces a hydrodynamic model explaining over-ionization in W49B and discusses open questions about mixed-morphology SNRs.

Findings

Over-ionized ejecta found in 3 mixed-morphology SNRs

Hydrodynamic model explains over-ionization in W49B

Open issues in understanding SNR evolution

Abstract

Mixed-morphology SNRs are characterized by a shell-like radio emission, a centrally peaked X-ray morphology, and by interaction with molecular clouds. Many models have been proposed to explain these peculiar remnants, but their physical origin is still unclear. The recent discovery of over-ionized (i. e. recombining) ejecta in 3 mixed-morphology SNRs has dramatically challenged all the previous models and opened up new, unexpected scenarios. I review the main properties of these remnants and their peculiar X-ray spectral properties. I also discuss the hydrodynamic model developed to explain the presence of over-ionized ejecta in W49B and present a list of open issues that still need to be clarified.