  Although observations of circumstellar shock interactions around supernovae
are generally consistent with a 1/r**2 density profile wind surrounding the
progenitor star, this is not true for GRB (gamma-ray burst) afterglows.
However, GRB 991208 and GRB 000301C may be consistent with wind interaction if
the injection particle spectrum is a broken power law. Circumstellar dust echos
can place constraints on supernova and GRB progenitors, but have been clearly
observed only around SN 1987A. Excess emission observed in two GRB afterglows
is more likely to have a supernova origin. An interstellar dust echo, causing
the light curve to flatten out, is a possibility for GRB afterglows, but is not
likely to be observable.
