A tremendous flare from SGR1806-20 with implications for short-duration gamma-ray bursts

Soft gamma-ray repeaters (SGRs) are X-ray stars which emit numerous short-duration (0.1 s) bursts of photons up to 100 keV during sporadic active periods. They are thought to be magnetars: neutron stars with observable emissions powered by magnetic dissipation. Here we report the detection of a rare 380 s long giant flare from SGR1806-20 on 27 December 2004, with energy greatly exceeding that of all previously-detected events. Its initial gamma-ray spike had a blackbody spectrum, characteristic of a relativistic pair/photon outflow. It carried away as much energy in 0.2 s as the Sun radiates in a quarter million years. This extreme energy suggests a catastrophic instability on a magnetar involving global crust failure and magnetic reconnection, perhaps with a significant large-scale untwisting of the magnetosphere. From a great distance this event would appear to be a short-duration, hard spectrum cosmic gamma-ray burst. We argue that this may partially explain the origin of one class of mysterious bursts. NASA's newly-commissioned Swift satellite is likely to detect extragalactic magnetars in significant numbers, opening up a new field of astronomical study