Discovery of strong radiative recombination continua from the supernova remnant IC 443 with suzaku

Abstract

We present the Suzaku spectroscopic study of the Galactic middle-aged supernova remnant (SNR) IC 443. The X-ray spectrum in the 1.75-6.0keV band is described by an optically thin thermal plasma with the electron temperature of ∼ 0.6keV 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 3.5keV. The ionization temperatures of Si and S determined from the intensity ratios of the RRC to He-like Kα lines are ∼ 1.0keV and ∼ 1.2keV, 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 was 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. © 2009. The American Astronomical Society. All rights reserved.