  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.
