ChandraX‐Ray Observation of a Mature Cloud‐Shock Interaction in the Bright Eastern Knot Region of Puppis A

We present Chandra X-ray images and spectra of the most prominent cloud-shockinteraction region in the Puppis A supernova remnant. The Bright Eastern Knot(BEK) has two main morphological components: (1) a bright compact knot thatlies directly behind the apex of an indentation in the eastern X-ray boundaryand (2) lying 1' westward behind the shock, a curved vertical structure (bar)that is separated from a smaller bright cloud (cap) by faint diffuse emission.Based on hardness images and spectra, we identify the bar and cap as a singleshocked interstellar cloud. Its morphology strongly resembles the ``voidedsphere'' structures seen at late times in Klein et al.'s experimentalsimulations of cloud-shock interactions, when the crushing of the cloud byshear instabilities is well underway. We infer an interaction time of roughly 3cloud-crushing timescales, which translates to 2000-4000 years, based on theX-ray temperature, physical size, and estimated expansion of the shocked cloud.This is the first X-ray identified example of a cloud-shock interaction in thisadvanced phase. Closer to the shock front, the X-ray emission of the compactknot in the eastern part of the BEK region implies a recent interaction withrelatively denser gas, some of which lies in front of the remnant. The complexspatial relationship of the X-ray emission of the compact knot to optical [OIII] emission suggests that there are multiple cloud interactions occurringalong the line of sight.Comment: 22 pages LaTeX with multiple figures, to appear in Ap