A First Look with Chandra at SGR 1806-20 after the Giant Flare: Significant Spectral Softening and Rapid Flux Decay

We report on the results of an ~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 ~2.2×10-11 ergs cm-2 s-1, about 20% lower than that measured 3 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 postflare X-ray flux decay of SGR 1806-20 has been much faster. The pulsed fraction was ~3%, a smaller value than that observed before the flare. We discuss the different properties of the postflare evolution of SGR 1806-20 in comparison to those of SGR 1900+14 and interpret the results as strong evidence that a magnetospheric untwisting occurred (or is occurring) after the giant flare.