Repeated injections of energy in the first 600 ms of the giant flare of SGR 1806–20


The massive flare of 27 December 2004 from the soft γ-ray repeater SGR 1806–20, a possible magnetar1,2,3, saturated almost all γ-ray detectors4,5,6,7, meaning that the profile of the pulse was poorly characterized. An accurate profile is essential to determine physically what was happening at the source. Here we report the unsaturated γ-ray profile for the first 600 ms of the flare, with a time resolution of 5.48 ms. The peak of the profile (of the order of 107 photons cm-2 s-1) was reached 50 ms after the onset of the flare, and was then followed by a gradual decrease with superposed oscillatory modulations possibly representing repeated energy injections with 60-ms intervals. The implied total energy is comparable to the stored magnetic energy in a magnetar ( 1047 erg) based on the dipole magnetic field intensity ( 1015 G), suggesting either that the energy release mechanism was extremely efficient or that the interior magnetic field is much stronger than the external dipole field2.

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Figure 1: Observed photon counts during the first 600 ms of the giant flare.
Figure 2: Calibration of the γ-ray sensitivity of the MCP detector.


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We thank R. Yamazaki, K. Ioka, K. Shibata, T. Minoshima and K. Hurley for comments and discussions. This work was supported in part by grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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Correspondence to Toshio Terasawa.

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Terasawa, T., Tanaka, Y., Takei, Y. et al. Repeated injections of energy in the first 600 ms of the giant flare of SGR 1806–20. Nature 434, 1110–1111 (2005).

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