The giant flare from SGR 1806-20 and its radio afterglow

The multi-wavelength observations of the 2004 December 27 Giant Flare (GF) from SGR 1806-20 and its long-lived radio afterglow are briefly reviewed. The GF appears to have been produced by a dramatic reconfiguration of the magnetic field near the surface of the neutron star, possibly accompanied by fractures in the crust. The explosive release of over 10⁴⁶ erg (isotropic equivalent) powered a one-sided mildly relativistic outflow. The outflow produced a new expanding radio nebula that is still visible over a year after the GF. Also considered are the constraints on the total energy in the GF, the energy and mass in the outflow, and on the external density, as well as possible implications for short γ-ray bursts and potential signatures in high energy neutrinos, photons, or cosmic rays. Some possible future observations of this and other GFs are briefly discussed.