XMM-Newton observations of unidentified INTEGRAL/IBIS sources

Abstract

About 30 per cent of the sources in the fourth INTEGRAL/IBIS catalogue are unidentified in that they lack an optical counterpart. To be able to classify them, X-ray observations are of crucial importance as they can place tighter constraints on the high-energy error box, which is usually of the order of a few arcminutes, and allow their broad-band spectrum to be studied. To this aim we have cross-correlated the list of all unidentified IBIS sources in the fourth catalogue with the archive of all XMM-Newton pointings, finding a set of six objects with archival data. For one of them, IGRJ17331-2406, no X-ray source is detected by XMM inside the IBIS error box, most likely due to the fact that it is a transient object. In the case of IGRJ17445-2747 two possible X-ray counterparts are found inside the IBIS error box: one is very weak while the other is bright, but only detected once. In each of the remaining four cases: IGRJ155359-5750, AXJ1739.3-2923, AXJ1740.2-2903 and IGRJ18538-0102, we find instead a convincing association, for which we provide an improved X-ray position and information on the optical/infrared counterpart. We also performed a detailed analysis of their XMM/IBIS spectra, and on the basis of all information acquired we suggest that IGRJ155359-5750 is an active galactic nucleus (AGN) of intermediate type, AXJ1739.3-2923 and AXJ1740.2-2903 are high-mass X-ray binary systems, IGRJ17331-2406 and IGRJ17445-2747 are Galactic transient sources and IGRJ18538-0102 could be a background AGN.