Discovery of a Radio Source following the 27 December 2004 Giant Flare from SGR 1806-20

Over a decade ago it was established that the remarkable high energy transients, known as soft gamma-ray repeaters (SGRs), are a Galactic population and originate from neutron stars with intense (<~ 10^15 G) magnetic fields ("magnetars"). On 27 December 2004 a giant flare (fluence >~ 0.3 erg/cm^2) was detected from SGR 1806-20. Here we report the discovery of a fading radio counterpart. We began a monitoring program from 0.2GHz to 250GHz and obtained a high resolution 21-cm radio spectrum which traces the intervening interstellar neutral Hydrogen clouds. Analysis of the spectrum yields the first direct distance measurement of SGR 1806-20. The source is located at a distance greater than 6.4 kpc and we argue that it is nearer than 9.8 kpc. If true, our distance estimate lowers the total energy of the explosion and relaxes the demands on theoretical models. The energetics and the rapid decay of the radio source are not compatible with the afterglow model that is usually invoked for gamma-ray bursts. Instead we suggest that the rapidly decaying radio emission arises from the debris ejected during the explosion.