XMM-Newton Observations of SGR 1806-20 in the Pre-Giant Flare Epoch
Dept. of Physics, University of Padova, Italy
I present results from XMM-Newton observations of the Soft Gamma-ray Repeater SGR 1806-20. Four pointings were carried out in 2003-2004 and catched the source in different states of
activity. During these two years the 2-10 keV flux doubled with respect to the historical level observed previously. The long term raise in luminosity was accompanied by a gradual hardening of the spectrum, with the power-law photon index decreasing from 2.2 to 1.5, and by a growth of the bursting activity. The pulse period measurements obtained in the four observations show a steady increase and are consistent with a high spin-down rate at an average value of 5.5e-10 s/s. The long term behavior of SGR 1806-20 follows the correlation between spectral hardness and spin-down rate
previously observed only by comparing different SGRs and AXPs. The relevance of these results for the magnetar scenario, and for the twisted magnetosphere model in particular, are discussed also in the light of post-flare Chandra and XMM observations.
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