A Rebrightening of the Radio Nebula Associated with the 2004 December 27 Giant Flare from SGR 1806-20

The 2004 Dec. 27 giant Gamma-ray flare detected from the magnetar SGR 1806-20created an expanding radio nebula which we have monitored with the AustraliaTelescope Compact Array and the Very Large Array. These data indicate thatthere was an increase in the observed flux ~25 days after the initial flarethat lasted for ~8 days, which we believe is the result of ambient materialswept-up and shocked by this radio nebula. For a distance to SGR 1806-20 of 15kpc, using the properties of this rebrightening we infer that the initial blastwave was dominated by baryonic material of mass M>10^{24.5} g. For an initialexpansion velocity v~0.7c (as derived in an accompanying paper), we infer thismaterial had an initial kinetic energy E>10^{44.5} ergs. If this materialoriginated from the magnetar itself, it may have emitted a burst of ultra-highenergy (E > 1 TeV) neutrinos far brighter than that expected from otherastrophysical sources.Comment: 7 pages, 2 figures. Accepted by ApJL, minor changes due to referee comment