An energetic blast wave from the December 27 giant flare of the soft gamma-ray repeater 1806-20

Soft gamma-ray repeaters (SGRs) distinguish themselves from classical gamma-ray bursts (GRBs) by their repetitive, soft bursts coming from nearby sources, likely strongly magnetized neutron stars dubbed ``magnetars''. Giant flares are rare events from SGRs, each characterized by an initial hard spike with more than a million Eddington luminosity and a pulsating tail persisting for hundreds of seconds. Recent follow-up observations of the December 27 giant flare of SGR 1806-20 have detected a multi-frequency radio afterglow from 240 MHz to 8.46 GHz, extending in time from one week to about one month after the flare. The angular size of the source was also measured for the first time. Here we show that this radio afterglow gives the first piece of clear evidence that an energetic blast wave sweeps up its surrounding medium and produces a synchrotron afterglow, the same mechanism as established for GRB afterglows. Future rapid radio/infrared/optical follow-ups of SGR giant flares and intermediate bursts will provide more detailed information of the early evolution of a fireball as well as its composition