A FIRST LOOK WITH CHANDRA AT SGR 1806-20 AFTER THE GIANT FLARE: SIGNIFICANT SPECTRAL SOFTENING AND RAPID FLUX DECAY

摘要

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 x 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.