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

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 ~10^36 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.