Recombining plasma in supernova remnants: Discovery and progress in the last decade

Presence of recombining (overionized) plasma in several supernova remnants (SNRs), first suggested by ASCA observations, was anchored by the detection of enhanced radiative recombination continua in Suzaku spectra of the SNRs IC 443 and W49B. This discovery was surprising, because X‑ray‑emitting SNRs were thought to have ionizing plasmas in general. Since then, similar spectral features have been detected from more than a dozen SNRs, implying that the recombining plasma is not uncommon for a certain type of SNRs. This paper reviews the observational and theoretical progress achieved in the last decade toward understanding of the origin of the recombining plasmas in SNRs. It is observationally indicated that all the SNRs whose plasma is in overionization are associated with the so‑called mixed‑morphology class, characterized by a centrally peaked X‑ray profile and rim‑brightened radio emission. The latest studies of W49B with NuSTAR and W44 with XMM‑Newton have provided clear evidence that two distinct mechanisms can work for the formation of the recombining plasma, namely adiabatic cooling and thermal conduction, with the dominant channel depending on the evolutionary stage of SNRs.