The calm after the storm: XMM-Newton observation of SGR 1806-20 two months after the Giant Flare of 2004 December 27 ⋆

XMM-Newtonobserved the soft gamma repeater SGR 1806-20 about two months after its 2004 December 27 giant flare. A comparison with the previous observations taken wit h the same instrument in 2003-2004 shows that the pulsed fraction and the spin-down rate have significantly decreased and that the spectrum slightly softened. These changes may indicate a global reconfiguration of the neutron star magnetosphere. The spec tral analysis confirms that the presence of a blackbody compo nent in addition to the power-law is required. Since this additio nal component is consistent with being constant with respect to the earlier observations, we explore the possibility of descri bing the long-term spectral evolution as only due to the power-law variations. In this case, the slope of the power-law does not significantly change and the spectral softening following t he giant flare is caused by the increase of the relative contribution o f the blackbody over the power-law component.