  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 > 8.8keV. We
confirm that the Fe RRC is the 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 work demonstrates NuSTAR's
suitability for studies of thermal emission, in addition to hard nonthermal
X-rays, from young and middle-aged SNRs.
