SIMBAD references

We report the results of an X-ray proper-motion measurement for the NW rim of SN 1006, carried out by comparing Chandra observations from 2001 to 2012. The NW limb has predominantly thermal X-ray emission, and it is the only location in SN 1006 with significant optical emission: a thin, Balmer-dominated filament. For most of the NW rim, the proper motion is ~0.''30/yr, essentially the same as has been measured from the Hα filament. Isolated regions of the NW limb are dominated by nonthermal emission, and here the proper motion is much higher, 0.''49/yr, close to the value measured in X-rays along the much brighter NE limb, where the X-rays are overwhelmingly nonthermal. At the 2.2 kpc distance to SN 1006, the proper motions imply shock velocities of ∼3000 km/s and ∼5000 km/s in the thermal and nonthermal regions, respectively. A lower velocity behind the Hα filament is consistent with the picture that SN 1006 is encountering denser gas in the NW, as is also suggested by its overall morphology. In the thermally dominated portion of the X-ray shell, we also see an offset in the radial profiles at different energies; the 0.5-0.6 keV peak dominated by O VII is closer to the shock front than that of the 0.8-3 keV emission–due to the longer times for heavier elements to reach ionization states where they produce strong X-ray emission.