Discovery of Strong Radiative Recombination Continua from the Supernova Remnant Ic 443 with Suzaku

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 Kα 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. Subject headings: ISM: individual (IC 443) — supernova remnants — radiation mechanisms: thermal — X-rays: ISM