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Fig. 17.7 Doppler maps of Si-K, S-K, and Fe-K emission lines in Cas A, extracted from XMM data . The surface brightness of the line emission is color coded with the Doppler velocity using the scheme shown in the bottom left panel confirmed the Markert et al. finding, but with substantially higher spatial detail. The X-ray emission geometry closely matches that observed in [S II]. Subsequent Chandra and XMM-Newton Doppler maps provided higher resolution views of the radial velocity structures [69, 178]. Willingale et al. used the XMM-Newton Doppler map to reconstruct a three-dimensional view of the Cas A X-ray emission (Fig. 17.7). They infer that the Si and S emission arises exclusively from ejecta and is confined to a narrow shell with radius 100-150 arc seconds. The Fe-K emission is confined in two large clumps, expanding faster and spanning a larger radius range of 110-170 arcsec. They liken these clumps to the ejecta bullets observed in Vela .
Flanagan et al. used the Chandra HETG to measure the radial velocities of the line emitting components in the SMC remnant 1E 0102.2-7219, along with their ionization structure (see Sect. 220.127.116.11) . By measuring the different distortions among the dispersed line images, they found Doppler shifts consistent with bulk velocities of—1000 km s^1. As for Cas A, the bulk velocities suggest that the emission is confined to a ring, inclined to the line of sight.
In contrast to these results, spatially resolved spectroscopy of Tycho indicates the absence of a significant systematic Doppler shift, suggesting that the remnant has a spherical geometry .
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