Giant Flare of SGR 1806-20 from a Relativistic Jet

The Japanese magnetospheric explorer, GEOTAIL, recorded a detailed light curve of the initial spike of a giant flare from SGR 1806-20 on 27 December 2004. We show that the observed light curve is well explained by emission from relativistically expanding fireballs, like those of gamma-ray bursts (GRBs). Especially, the observed rapid fading after 500 msec suggests that ejecta was collimated in a jet. We derived an upper limit on the jet opening halfangle of 0.2 rad, which is as narrow as those of GRBs. Observed proper motion of a centroid of a radio image also suggests the collimated outflow. Possible way to avoid the ''statistical'' problem is also discussed.