Two years of INTEGRAL monitoring of the soft gamma-ray repeater SGR 1806-20: from quiescence to frenzy

¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, via Bassini 15, 20133 Milano, Italy e-mail: diego@mi.iasf.cnr.it
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
³ Max Planck Institut für Astrophysik, Karl Schwarzschild Str. 1, 85740 Garching bei München, Germany
⁴ University of California at Berkeley, Space Sciences Laboratory, Berkeley CA 94720-7450, USA
⁵ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
⁶ External ESA Fellow – Centre d'Étude Spatiale des Rayonnements (CESR), avenue du Colonel-Roche 9, 31028 Toulouse Cedex 4, France
⁷ Danish National Space Center (DNSC), Juliane Maries Vej 30, 2100 Copenhagen, Danemark
⁸ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, via Fosso del Cavaliere 100, 00133 Roma, Italy
⁹ Sabanci University, Orhanli-Tuzla, Istanbul, Turkey
¹⁰ Astronomical Institute “Anton Pannekoek”, University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
¹¹ National Space Science and Technology Center, 320 Sparkman Drive, Huntsville, AL 35805, USA
¹² NASA Marshall Space Flight Center, XD 12, Huntsville, AL 35812, USA
¹³ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, via Gobetti 101, 40129 Bologna, Italy
¹⁴ ESA-ESTEC, RSSD, Keplerlaan 1, 2201 AZ Nordwijk, The Netherlands
¹⁵ Department of Physics, University of Manchester, Sackville Street, PO Box 88, Manchester M60 1QD, UK

Received: 17 June 2005
Accepted: 29 August 2005

Abstract

SGR 1806–20 has been observed for more than 2 years with the INTEGRAL satellite. In this period the source went from a quiescent state into a very active one culminating in a giant flare on December 27, 2004. Here we report on the properties of all the short bursts detected with INTEGRAL before the giant flare. We derive their number-intensity distribution and confirm the hardness-intensity correlation for the bursts found by Götz et al. (2004a, A&A, 417, L45). Our sample includes a very bright outburst that occurred on October 5, 2004, during which over one hundred bursts were emitted in 10 minutes, involving an energy release of erg. We present a detailed analysis of it and discuss our results in the framework of the magnetar model.