The broad-band spectrum of the persistent emission from SGR 1806-20

Abstract

We present the results of an analysis of the quiescent X-ray emission from the Soft Gamma-Ray Repeater SGR 1806-20, taken during an INTEGRAL ultra-deep survey of the Galactic center region in Aug.-Sep. 2003. The total effective exposure time spent on the source by the IBIS telescope during these observations was 1.6 million seconds. Combining the INTEGRAL results with results from the XMM-Newton observatory, we present the broad band (1-200 keV) spectrum of the quiescent emission from this source. This is the first spectrum of the persistent emission from an SGR in the broad energy range up to 200 keV. The spectrum of the source is very hard and has a power law shape ($\Gamma=1.6\pm0.1$) without any trace of a high energy cutoff up to ~160 keV. No strong cyclotron line was detected in the persistent spectrum. The luminosity of SGR 1806-20 in this range (1-200 keV) was ${\sim} 3.6\times10^{36}$ erg s -1 for an assumed distance of 15 kpc. We show that weak undetected bursts should not contribute significantly to the quiescent emission. During our next observations in August 2004 the source went into an active phase and its average flux between powerful bursts was 2-3 times higher than in 2003. During Aug.-Sep. 2003, two other SGR candidates, SGR 1801-23 and SGR 1808-20, were in the field of view. Neither persistent hard X-ray emission nor bursts were detected from them. The upper limit on the persistent flux from each of them in the energy band 18-100 keV is about $4\times10^{-11}$ erg s -1 cm -2 .