A rebrightening of the radio nebula associated with the 2004 December 27 giant flare from SGR 1806-20

The 2004 December 27 giant gamma-ray flare detected from the magnetar SGR 1806 - 20 created an expanding radio nebula that 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 similar to 25 days after the initial flare that lasted for similar to 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 greater than or similar to 10(24.5) g. For an initial expansion velocity v similar to 0.7c derived in an accompanying paper), we infer that this material had an initial kinetic energy E greater than or similar to 10(44.5) ergs. If this material originated from the magnetar itself, it may have emitted a burst of ultra - high-energy (E greater than or similar to 1 TeV) neutrinos far brighter than that expected from other astrophysical sources.