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A first look with Chandra at SGR 1806-20 after the Giant Flare: significant spectral softening and rapid flux decay

N. Rea·A. Tiengo·S. Mereghetti·G. L. Israel·S. Zane·R. Turolla·L. Stella
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Abstract

We report on the results of a ~30 ks Chandra pointing of the soft gamma-ray repeater SGR 1806-20, the first X-ray observation with high spectral resolution performed after the 2004 December 27 Giant Flare. The source was found in a bursting active phase and with a significantly softer spectrum than that of the latest observations before the Giant Flare. The observed flux in the 2-10 keV range was ~2.2 x10^-11 erg/s/cm^2, about 20% lower than that measured three months before the event. This indicates that, although its giant flare was ~100 times more intense than those previously observed in two other soft gamma-ray repeaters, the post flare X-ray flux decay of SGR 1806-20 has been much faster. The pulsed fraction was about 3%, a smaller value than that observed before the flare. We discuss the different properties of the post-flare evolution of SGR 1806-20 in comparison to those of SGR 1900+14 and interpret the results as a strong evidence that a magnetospheric untwisting occurred (or is occurring) after the Giant Flare.

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N. Rea, A. Tiengo, S. Mereghetti, G. L. Israel, S. Zane, R. Turolla, L. Stella. 2005-05-20. A first look with Chandra at SGR 1806-20 after the Giant Flare: significant spectral softening and rapid flux decay. https://doi.org/10.1086/431951

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