SGR 1806-20 about two years after the giant flare: Suzaku, XMM-Newton and INTEGRAL observations

¹ Università degli Studi di Pavia, Dipartimento di Fisica Nucleare e Teorica and INFN-Pavia, via Bassi 6, 27100 Pavia, Italy e-mail: paoloesp@iasf-milano.inaf.it
² INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via Bassini 15, 20133 Milano, Italy
³ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking Surrey, RH5 6NT, UK
⁴ Università di Padova, Dipartimento di Fisica, via Marzolo 8, 35131 Padova, Italy
⁵ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique, 91191 Gif-sur-Yvette, France
⁶ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁷ University of Amsterdam, Astronomical Institute “Anton Pannekoek”, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
⁸ Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
⁹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
¹⁰ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Roma, via del Fosso del Cavaliere 100, 00133 Rome, Italy

Received: 28 August 2007
Accepted: 8 October 2007

Abstract

In December 2004, the soft gamma-ray repeater SGR 1806-20 emitted the most powerful giant flare ever observed. This probably involved a large-scale rearrangement of the magnetosphere leading to observable variations in the properties of its X-ray emission. Here we present the results of the first Suzaku observation of SGR 1806-20, together with almost simultaneous observations with XMM-Newton and INTEGRAL. The source seems to have reached a state characterized by a flux close to the pre-flare level and by a relatively soft spectrum. Despite this, SGR 1806-20  also remained quite active after the giant flare, allowing us to study several short bursts observed by Suzaku in the 1–100 keV range. We discuss the broad-band spectral properties of SGR 1806-20, covering both persistent and bursting emission, in the context of the magnetar model, and consider its recent theoretical developments.