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

¹ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191, Gif-sur-Yvette, France e-mail: mfalanga@cea.fr
² Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
³ IASF - CNR, Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy
⁴ Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy

Received: 6 May 2004
Accepted: 6 July 2004

Abstract

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 ~ erg s^-1 in the 3–20 keV energy band and ~ erg s^-1 in the 20–100 keV energy range, for a distance of 8 kpc.