  The 2004 Dec. 27 giant Gamma-ray flare detected from the magnetar SGR 1806-20
created an expanding radio nebula which we have monitored with the Australia
Telescope Compact Array and the Very Large Array. These data indicate that
there was an increase in the observed flux ~25 days after the initial flare
that lasted for ~8 days, which we believe is the result of ambient material
swept-up and shocked by this radio nebula. For a distance to SGR 1806-20 of 15
kpc, using the properties of this rebrightening we infer that the initial blast
wave was dominated by baryonic material of mass M>10^{24.5} g. For an initial
expansion velocity v~0.7c (as derived in an accompanying paper), we infer this
material had an initial kinetic energy E>10^{44.5} ergs. If this material
originated from the magnetar itself, it may have emitted a burst of ultra-high
energy (E > 1 TeV) neutrinos far brighter than that expected from other
astrophysical sources.
