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

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.