Letter to the Editor

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

¹ NAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Milano “G. Occhialini”, via Bassini 15, 20133 Milano, Italy e-mail: tiengo@mi.iasf.cnr.it
² Università degli Studi di Milano, Dipartimento di Fisica, via Celoria 16, 20133 Milano, Italy
³ INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Roma, Italy
⁴ SRON – National Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
⁵ Università di Padova, Dipartimento di Fisica, via Marzolo 8, 35131 Padova, Italy
⁶ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking Surrey, RH5 6NT, UK

Received: 1 July 2005
Accepted: 4 August 2005

Abstract

XMM-Newton observed the soft gamma repeater SGR 1806–20  about two months after its 2004 December 27 giant flare. A comparison with the previous observations taken with 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 spectral analysis confirms that the presence of a blackbody component in addition to the power-law is required. Since this additional component is consistent with being constant with respect to the earlier observations, we explore the possibility of describing 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 the giant flare is caused by the increase of the relative contribution of the blackbody over the power-law component.