Suzaku Discovery of the Strong Radiative Recombination Continuum of Iron from the Supernova Remnant W49B

Abstract

We present a hard X-ray spectrum of unprecedented quality of the Galactic supernova remnant (SNR) W49B obtained with the Suzaku satellite. The spectrum exhibits an unusual structure consisting of a saw-edged bump above 8 keV. This bump cannot be explained by any combination of high-temperature plasmas in ionization equilibrium. We firmly conclude that this bump is caused by the strong radiative recombination continuum (RRC) of iron, detected for the first time in a SNR. The electron temperature derived from the bremsstrahlung continuum shape and the slope of the RRC is ~1.5 keV. On the other hand, the ionization temperature derived from the observed intensity ratios between the RRC and Kα lines of iron is ~2.7 keV. These results indicate that the plasma is in a highly overionized state. Volume emission measures independently determined from the fluxes of the thermal and RRC components are consistent with each other, suggesting the same origin of these components.