Chandra and XMM-Newton have made possible detailed spatial-spectral analysis of shock structures in nearby, evolved SNRs, allowing comparison with shock simulations, and in one case, ground-based experiments.
High-resolution Chandra observations of portions of the Cygnus Loop shell reveal enormous complexity, and multiple emission structures. Regions along the southeastern and western rims have been observed. In the west, the interaction is observed between the shock front and both a large interstellar cloud and the shell that surrounds the cavity formed by the progenitor's wind . The X-rays uniquely trace the shock front within the dense cloud. The temperature where the shock front encounters the cloud is very low, —0.03 keV. Behind this shock front the temperature is higher, 0.2 keV, due to the reheating of material by a reflected shock. The interaction of the shock with the cavity boundary creates a thin (0.1 pc) X-ray structure showing clear radial temperature variation. The temperature at the outer shock is 0.12 keV; the slightly higher temperature material behind this region is also ascribed to a reflected shock. No evidence for turbulence associated with instabilities is detected along either shock structure, nor are anomalous metal abundances. The southeastern knot is thought to represent the early stage of an encounter between the blast wave and a large interstellar cloud. It appears to have an extremely complicated morphology, but the Chandra observations help demonstrate that the apparent complexity is due primarily to projection of multiple thin, fairly simple shock structures .
The Eastern Bright Knot (EBK) of the 4 000-year-old remnant Puppis A (at a distance of —2kpc) was identified using the Einstein HRI as the site of the collision between the SNR shock and an isolated interstellar cloud. Subsequent radio and optical observations suggested that the EBK is related to a more extended interstellar feature, and that only a small portion has been encountered by the Puppis A
Fig. 17.13 The complex structure of the Eastern Bright Knot in Puppis A revealed by ACIS indicates it consists of two distinct features, a fully-shocked isolated cloud (right) and the result of more recent interaction with dense gas . The soft band (O) is in red, medium band (Ne) is in green, and hard band (>1.5 keV) is in blue
Fig. 17.13 The complex structure of the Eastern Bright Knot in Puppis A revealed by ACIS indicates it consists of two distinct features, a fully-shocked isolated cloud (right) and the result of more recent interaction with dense gas . The soft band (O) is in red, medium band (Ne) is in green, and hard band (>1.5 keV) is in blue shock. A spatial-spectral analysis using the Chandra ACIS indicates the EBK is two distinct features, with very different temperature structures . These are apparent in Fig. 17.13. The inner of these is probably an isolated cloud; its morphology is strikingly similar to the results of laboratory simulation. Comparison with these simulations leads to inference of an interaction time of roughly three cloud-crushing timescales, which translates to 2 000-4 000 yrs. This is the first X-ray identified example of a cloud-shock interaction in this advanced phase. The X-ray emission of the compact knot, located approximately an arc minute closer to the edge of the remnant, implies a recent interaction with relatively denser gas, some of which is located in front of the remnant. Observations in other bands indicate that the gas cloud has a complicated morphology.
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