Spatially resolved ROSAT X-ray and ground-based optical data for the southwestern region of the Cygnus Loop SNR reveal in unprecedented detail the very early stages of a blast wave interaction with an isolated interstellar cloud .
Numerous internal cloud shock fronts near the upstream flow and along the cloud edges are visible optically as sharp filaments of enhanced H \alpha emission .
Faint X-ray emission is seen along a line of Balmer-dominated shock filaments north and south of the cloud with an estimated X-ray gas temperature of 1.2 \times 10 ^ { 6 } K ( 0.11 keV ) corresponding to a shock velocity of 290 km s ^ { -1 } .
The main cloud body itself exhibits little or no X-ray flux .
Instead , X-ray emission is confined along the northern and southernmost cloud edges , with the emission brightest in the downstream regions farthest from the shock front ’ s current position .
We estimate an interaction age of \sim 1200 yr based on the observed shock/cloud morphology .

Overall , the optical and X-ray properties of this shocked ISM cloud show many of the principal features predicted for a young SNR shock – ISM cloud interaction .
In particular , one sees shocklet formation and diffraction inside the inhomogenous cloud along with partial main blast wave engulfment .
However , several significant differences from model predictions are also present including no evidence for turbulence along cloud edges , diffuse rather than filamentary [ O iii ] emission within the main body of the cloud , unusually strong downstream [ S ii ] emission in the postshock cloud regions , and confinement of X-ray emission to the cloud ’ s outer boundaries .