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An exceptionally bright flare from SGR 1806–20 and the origins of short-duration γ-ray bursts


Soft-γ-ray repeaters (SGRs) are galactic X-ray stars that emit numerous short-duration (about 0.1 s) bursts of hard X-rays during sporadic active periods. They are thought to be magnetars: strongly magnetized neutron stars with emissions powered by the dissipation of magnetic energy. Here we report the detection of a long (380 s) giant flare from SGR 1806–20, which was much more luminous than any previous transient event observed in our Galaxy. (In the first 0.2 s, the flare released as much energy as the Sun radiates in a quarter of a million years.) Its power can be explained by a catastrophic instability involving global crust failure and magnetic reconnection on a magnetar, with possible large-scale untwisting of magnetic field lines outside the star. From a great distance this event would appear to be a short-duration, hard-spectrum cosmic γ-ray burst. At least a significant fraction of the mysterious short-duration γ-ray bursts may therefore come from extragalactic magnetars.

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Figure 1: Profiles of the 27 December 2004 giant flare.
Figure 2: Spectrum and time history of the initial spike, from the RHESSI and Wind particle detectors.
Figure 3: Time-averaged counts in the tail phase of the giant flare, compared with the ‘trapped fireball’ model.
Figure 4: 3–100-keV phase-averaged energy spectrum of the pulsed tail, from the RHESSI γ-ray detectors.
Figure 5: Detailed profiles of the oscillations, from the RHESSI γ-ray detectors.

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We are grateful to J. Scalo, E. Vishniac and S. Kannappan for discussions and expert help. In the US, this work was supported by NASA. The INTEGRAL mission is supported by the German government via the DLR agency.

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Correspondence to K. Hurley.

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Supplementary information

Supplementary Notes

This contains information on the detectors, their count rates, their dead times, and other details of the observations. It also explains the details of the calculations used to derive the detectability and rate of magnetar bursts. (DOC 32 kb)

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Hurley, K., Boggs, S., Smith, D. et al. An exceptionally bright flare from SGR 1806–20 and the origins of short-duration γ-ray bursts. Nature 434, 1098–1103 (2005).

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