The discovery of hard X–ray emission in the persistent flux of the Rapid Burster

We report the first detection with INTEGRAL of persistent hard X–ray emission (20 to 100 keV) from the Rapid Burster (MXB 1730–335), and describe its full spectrum from 3 to 100 keV. The source was detected on February/March 2003 during one of its recurrent outbursts. The source was clearly detected with a high signal to noise ratio during the single pointings and is well distinguished from the neighboring source GX 354–0. The 3 – 100 keV X–ray spectrum of the persistent emission is well described by a two-component model consisting of a blackbody plus a power-law with photon index ∼ 2.4. The estimated luminosity was ∼ 8.5× 10 erg s in the 3 – 20 keV energy band and ∼ 1.3 × 10 erg s in the 20 – 100 keV energy range, for a distance of 8 kpc.