  Recent observations have shown several supernova remnants (SNRs) have
overionized plasmas, those where ions are stripped of more electrons than they
would be if in equilibrium with the electron temperature. Rapid electron
cooling is necessary to produce this situation, yet the physical origin of that
cooling remains uncertain. To assess the cooling scenario responsible for
overionization, in this paper, we identify and map the overionized plasma in
the Galactic SNR W49B based on a 220 ks Chandra Advanced CCD Imaging
Spectrometer (ACIS) observation. We performed a spatially-resolved
spectroscopic analysis, measuring the electron temperature by modeling the
continuum and comparing it to the temperature given by the flux ratio of the
He-like and H-like lines of sulfur and of argon. Using these results, we find
that W49B is overionized in the west, with a gradient of increasing
overionization from east to west. As the ejecta expansion is impeded by
molecular material in the east but not in the west, our overionization maps
suggest the dominant cooling mechanism is adiabatic expansion of the hot
plasma.
