A Re-brightening of the Radio Nebula Associated with the 2004 December 27 Giant Flare from Sgr 1806–20

On 2004 Dec. 27, a giant γ-ray flare was detected from the magnetar SGR 1806–20. A radio observation seven days later revealed an expanding radio nebula at this position. Here we present results from an on-going monitoring campaign of this source with the Australia Telescope Compact Array and Very Large Array. These data indicate that there was an increase in the observed flux ∼25 days after the initial γ-ray flare that lasted for ∼3–5 days. In this Letter, we argue that this rebrightening marks the end of the coasting phase of the blast wave and the transition to the Sedov-Taylor phase. Assuming a distance to SGR 1806–20 of 15 kpc, we infer from the properties of this rebrightening that the blast wave is baryonic material of mass M & 1024.5 g, initially expanding with a velocity of about 0.3c, and therefore has a kinetic energy E & 1044 ergs. If this mass was blown off the outer layers of the magnetar, it may have emitted a burst of ultra-high energy (E > 1 TeV) neutrinos far in excess of what might be expected from other astrophysical sources. Subject headings: neutrinos — pulsars: individual (SGR 1806–20) — radio continuum: stars — shock waves — stars: magnetic fields, neutron