  Soft gamma repeaters (SGRs) are "magnetars", a small class of slowly spinning
neutron stars with extreme surface magnetic fields, ~10^15 gauss. On 2004
December 27, a giant flare was detected from the magnetar SGR 1806-20, the
third such event ever recorded. This burst of energy was detected by a variety
of instruments and even caused an ionospheric disturbance in the Earth's upper
atmosphere recorded around the globe. Here we report the detection of a fading
radio afterglow produced by this outburst, with a luminosity 500 times larger
than the only other detection of a similar source. From day 6 to day 19 after
the flare from SGR 1806-20, a resolved, linearly polarized, radio nebula was
seen, expanding at approximately a quarter the speed of light. To create this
nebula, at least 4x10^43 ergs of energy must have been emitted by the giant
flare in the form of magnetic fields and relativistic particles. The
combination of spatially resolved structure and rapid time evolution allows a
study in unprecedented detail of a nearby analog to supernovae and gamma-ray
bursts.
