Time Evolution of the Reverse Shock in SN 1006
  The Schweizer-Middleditch star, located behind the SN 1006 remnant and near
its center in projection, provides the opportunity to study cold, expanding
ejecta within the SN 1006 shell through UV absorption. Especially notable is an
extremely sharp red edge to the Si II 1260 Angstrom feature, which stems from
the fastest moving ejecta on the far side of the SN 1006 shell--material that
is just encountering the reverse shock. Comparing HST far-UV spectra obtained
with COS in 2010 and with STIS in 1999, we have measured the change in this
feature over the intervening 10.5-year baseline. We find that the sharp red
edge of the Si II feature has shifted blueward by 0.19 +/- 0.05 Angstroms,
which means that the material hitting the reverse shock in 2010 was moving
slower by 44 +/- 11 km/s than the material that was hitting it in 1999, a
change corresponding to - 4.2 +/- 1.0 km/s/yr. This is the first observational
confirmation of a long-predicted dynamic effect for a reverse shock: that the
shock will work its way inward through expanding supernova ejecta and encounter
ever slower material as it proceeds. We also find that the column density of
shocked Si II (material that has passed through the reverse shock) has
decreased by 7 +/- 2% over the ten-year period. The decrease could indicate
that in this direction the reverse shock has been ploughing through a dense
clump of Si,leading to pressure and density transients.
