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

TL;DR

This paper reports the first detection of a strong radiative recombination continuum of iron in a supernova remnant, revealing an overionized plasma state with unprecedented spectral quality from Suzaku observations.

Contribution

It provides the first clear evidence of iron RRC in a supernova remnant and characterizes the plasma's overionized state using high-quality X-ray spectra.

Findings

Detection of iron RRC in W49B for the first time.

Plasma is highly overionized with electron temperature 1.5 keV and ionization temperature 2.7 keV.

Consistent volume emission measures support a common origin of thermal and RRC components.

Abstract

We present a hard X-ray spectrum of unprecedented quality of the Galactic supernova remnant 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 supernova remnant. 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 K-alpha 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.