X-ray Proper Motions and Shock Speeds along the Northwest Rim of SN 1006
  We report the results of an X-ray proper motion measurement for the NW rim of
SN1006, carried out by comparing Chandra observations from 2001 and 2012. The
NW limb has predominantly thermal X-ray emission, and it is the only location
in SN1006 with significant optical emission: a thin, Balmer-dominated filament.
For most of the NW rim, the proper motion is about 0.30 arcsec/yr, essentially
the same as has been measured from the H-alpha filament. Isolated regions of
the NW limb are dominated by nonthermal emission, and here the proper motion is
much higher, 0.49 arcsec/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 SN1006, the proper motions imply shock velocities of about
3000 km/s and 5000 km/s in the thermal and nonthermal regions, respectively. A
lower velocity behind the H-alpha filament is consistent with the picture that
SN1006 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.
