The gamma-ray giant flare from SGR 1806-20: Evidence of crustal cracking via initial timescales

Soft gamma-ray repeaters (SGRs) are neutron stars that emit short (<= 1 s) and energetic (<= 10(42) ergs s(-1)) bursts of soft gamma-rays. Only four of them are currently known. Occasionally, SGRs have been observed to emit much more energetic "giant flares" (similar to 10(44)-10(45) ergs s(-1)). These are exceptional and rare events. We report here on serendipitous observations of the intense gamma-ray flare from SGR 1806-20 that occurred on 2004 December 27. Unique data from the Cluster and Double Star TC-2 satellites, designed to study the Earth's magnetosphere, provide the first observational evidence of three separate timescales within the early (first 100 ms) phases of this class of events. These observations reveal that in addition to the initial very steep (< 0.25 ms) X-ray onset, there is first a 4.9 ms exponential rise timescale followed by a continued exponential rise in intensity on a timescale of 70 ms. These three timescales are a prominent feature of current theoretical models, including the timescale (several milliseconds) for fracture propagation in the crust of the neutron star.