Discovery of Strong Radiative Recombination Continua from The Supernova Remnant IC 443 with Suzaku

TL;DR

This study uses Suzaku X-ray observations to discover strong radiative recombination continua in the supernova remnant IC 443, providing evidence for an overionized plasma likely formed during early dense phases and cooled by expansion.

Contribution

First detection of strong RRC features in IC 443, indicating an overionized plasma state and challenging previous thermal conduction explanations.

Findings

Detection of strong RRC of H-like Si and S.

Ionization temperatures of Si and S are 1.0 keV and 1.2 keV.

Evidence for an overionized, recombining plasma.

Abstract

We present the Suzaku spectroscopic study of the Galactic middle-aged supernova remnant (SNR) IC 443. The X-ray spectrum in the 1.75-6.0 keV band is described by an optically-thin thermal plasma with the electron temperature of 0.6 keV and several additional Lyman lines. We robustly detect, for the first time, strong radiative recombination continua (RRC) of H-like Si and S around at 2.7 and 3.5 keV. The ionization temperatures of Si and S determined from the intensity ratios of the RRC to He-like K-alpha line are 1.0 keV and 1.2 keV, respectively. We thus find firm evidence for an extremely-overionized (recombining) plasma. As the origin of the overionization, a thermal conduction scenario argued in previous work is not favored in our new results. We propose that the highly-ionized gas were made at the initial phase of the SNR evolution in dense regions around a massive progenitor, and the low electron temperature is due to a rapid cooling by an adiabatic expansion.