Letter to the Editor

INTEGRAL discovery of persistent hard X-ray emission from the Soft Gamma-ray Repeater SGR 1806–20 ^*

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica – Sezione di Milano “G. Occhialini” – INAF, via Bassini 15, 20133 Milano, Italy e-mail: sandro@mi.iasf.cnr.it
² European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
³ University of California at Berkeley, Space Sciences Laboratory, Berkeley, CA 94720-7450, USA

Received: 26 November 2004
Accepted: 27 December 2004

Abstract

We report the discovery of persistent hard X-ray emission extending up to 150 keV from the soft gamma-ray repeater SGR 1806–20 using data obtained with the INTEGRAL satellite in 2003–2004. Previous observations of hard X-rays from objects of this class were limited to short duration bursts and rare transient episodes of strongly enhanced luminosity (“flares”). The emission observed with the IBIS instrument above 20 keV has a power law spectrum with photon index in the range 1.5–1.9 and a flux of 3 milliCrabs, corresponding to a 20–100 keV luminosity of ~10³⁶ erg s^-1 (for a distance of 15 kpc). The spectral hardness and the luminosity correlate with the level of source activity as measured from the number of emitted bursts.