Evidence for Rapid Adiabatic Cooling as an Origin of the Recombining Plasma in the Supernova Remnant W49B Revealed by NuSTAR Observations

X-ray observations of supernova remnants (SNRs) in the last decade have shown that the presence of recombining plasmas is somewhat common in a certain type of object. The SNR W49B is the youngest, hottest, and most highly ionized among such objects, and hence provides crucial information about how the recombination phase is reached during the early evolutionary phase of SNRs. In particular, spectral properties of radiative recombination continuum (RRC) from Fe are the key for constraining the detailed plasma conditions. Here we present imaging and spectral studies of W49B with Nuclear Spectroscopic Telescope Array (NuSTAR), utilizing the highest-ever sensitivity to the Fe RRC at ≳8.8 keV. We confirm that the Fe RRC is most prominent at the western part of the SNR because of the lowest electron temperature (∼1.2 keV) achieved there. Our spatially resolved spectral analysis reveals a positive correlation between the electron temperature and the recombination timescale with a uniform initial temperature of ∼4 keV, which is consistent with the rapid adiabatic cooling scenario as an origin of the overionization. This Letter demonstrates NuSTAR's suitability for studies of thermal emission, in addition to hard nonthermal X-rays, from young and middle-aged SNRs.