Using the ROSAT Bright Source Catalogue to find Counterparts for IBIS/ISGRI Survey Sources

The IBIS/ISGRI first year galactic plane survey has produced a catalogue containing 123 hard X-ray sources visible down to a flux limit of a few milliCrabs. The point source location accuracy of typically 1-3 arcminutes has allowed the counterparts for 95 of these sources to be found at other wavelengths. In order to identify the remaining 28 objects, we have cross-correlated the ISGRI catalogue with the ROSAT All Sky Survey Bright Source Catalogue. In this way, for ISGRI sources which have a counterpart in soft X-rays, we can use the, much smaller, ROSAT error box to search for identifications. As expected, we find a strong correlation between the two catalogues and calculate that there are 75 associations with the number expected by chance to be almost zero. Of these 75 sources, ten are in the list of unidentified objects. Using the ROSAT error boxes we provide tentative associations for 8 of these.