Kinematic Evidence for Bipolar Ejecta Flows in the Galactic SNR W49B

W49B is a unique Galactic supernova remnant with centrally peaked, "bar"-like ejecta distribution, which was once considered evidence for a hypernova origin that resulted in a bipolar ejection of the stellar core. However, chemical abundance measurements contradict this interpretation. Closely connected to the morphology of the ejecta is its velocity distribution, which provides critical details for understanding the explosion mechanism. We report the first-ever observational constraint on the kinematics of the ejecta in W49B using the Resolve microcalorimeter spectrometer on the X-ray Imaging and Spectroscopy Mission (XRISM). Using XRISM/Resolve, we measured the line-of-sight velocity traced by the Fe He$\alpha$ emission, which is the brightest feature in the Resolve spectrum, to vary by $\pm$300 km s$^{-1}$ with a smooth east-to-west gradient of a few tens of km s$^{-1}$ pc$^{-1}$ along the major axis. Similar trends in the line-of-sight velocity structure were found for other Fe-group elements Cr and Mn, traced by the He$\alpha$ emission, and also for intermediate-mass elements Si, S, Ar, and Ca, traced by the Ly$\alpha$ emission. The discovery of the east-west gradient in the line-of-sight velocity, together with the absence of a twin-peaked line profile or enhanced broadening in the central region, clearly rejects the equatorially expanding disk model. In contrast, the observed velocity structure suggests bipolar flows reminiscent of a bipolar explosion scenario. An alternative scenario would be a collimation of the ejecta by an elongated cavity sculpted by bipolar stellar winds.