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We have used the ROSAT high resolution imager and optical emission-line data to study the structure of a bright optical knot on the southeastern rim of the Cygnus Loop supernova remnant. This knot has been identified as an encounter between the blast wave and a small, isolated cloud. The knot appears in projection just behind the blast wave, which is traced by Balmer line filaments which bound the X-ray emission. The knot is a prominent X-ray feature, consisting of a number of filaments which are correlated with the optical line emission. These data permit a detailed view of the blast wave interaction. By combining the optical and X-ray data is is possible to trace the blast wave as a continuous surface from its southern edge around the western side of the cloud which then continuing on to the north. The SE knot is an indentation on the surface of the blast wave. Thus the SE knot is not a small cloud that has been overrun by the blast wave, but the tip of a larger cloud.
Bright X-ray emission is associated with bright radiative filaments. The location of this emission, upsteam of the radiative shocks, implies that the enhanced X-rays comes from a reverse shock. The presence of a reverse shock is further evidence that the SE knot represents a an early stage of a blast wave encountering a large cloud.
A consistent picture of the Cygnus Loop as an explosion within a preexising cavity is emerging. These observations agree with that picture.
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