Discovery of strong radiative recombination continua from IC443

We present the Suzaku detection of strong free-bound emission from the Galactic middle-aged supernova remnant (SNR) IC 443. A previous ASCA observation revealed that the Lyalfpha fluxes of Si and S are significantly higher than those expected from the electron temperature of the bremsstrahlung continuum (Kawasaki et al. 2002). Therefore, it was claimed that the thermal plasma in this SNR is in "overionization". In the XIS spectrum of IC 443, we discovered hump-like features around at 2.7 and 3.5 keV as well as the strong Lyalpha lines. These humps are well represented by radiative recombination continua (RRC) of H-like Si and S with the electron temperature of approximately 0.6 keV. The ionization temperatures of Si and S determined from the intensity ratios of the RRC to He-like Kalpha line are approximately 1.0 keV and approximately 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 the 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.
Meeting Information: The 3rd Suzaku international Conference "Energetic Cosmos : from Suzaku to ASTRO-H" (June 29-July 2, 2009. Grand Park Otaru Hotel), Otaru, Hokkaido Japan
Meeting sponsors: The University of Tokyo, The Institute of Physical and Chemical Research, The Japan Society for the Promotion of Science