Swift/XRT observations of unidentified INTEGRAL/IBIS sources

Abstract

The most recent IBIS/ISGRI survey, i.e. the fourth one, lists 723 hard X-ray sources many still unidentified, i.e. lacking an X-ray counterpart or simply not studied at lower energies, i.e. below 10 keV. In order to overcome this lack of X-ray information, we cross-correlated the list of IBIS sources included in the fourth IBIS catalogue with the Swift/XRT data archive, finding a sample of 20 objects, not yet reported in the literature, for which XRT data could help in the search for the X-ray and hence optical counterpart and/or, for the first time, in the study of the source spectral and variability properties below 10 keV. Sixteen of these objects are new INTEGRAL detections, while four were already listed in the third survey but not yet observed in X-rays. Four objects (IGR J00465–4005, LEDA 96373, IGR J1248.2–5828 and IGR J13107–5626)are confirmed or likely absorbed active galaxies, while two (IGR J14080– 3023 and 1RXS J213944.3+595016) are unabsorbed AGN. We also find three peculiar extragalactic objetcs, NGC 4728 being a Narrow Line Seyfert galaxy, MCG+04–26– 006 a type 2 LINER and PKS 1143–693 probably a QSO; furthermore, our results indicate that IGR J08262+4051 and IGR J22234–4116 are candidate AGN, which require further optical spectroscopic follow-up observations to be fully classified. Only in the case of 1RXS J080114.6–462324 we are confident that the source is a Galactic object. For IGR J10447–6027, IGR J12123–5802 and IGR J20569+4940 we pinpoint one X-ray counterpart, although its nature could not be assessed despite spectral and sometimes variability information being obtained. Clearly, we need to perform optical follow-up observations in order to firmly assess their nature. There are five objects for which we find no obvious X-ray counterpart (IGR J07506–1547 and IGR J17008–6425) or even no detection (IGR J17331–2406, IGR J18134–1636 and IGR J18175–1530); apart from IGR J18134–1636, all these sources are found to be variable in the IBIS energy band, therefore it is difficult to catch themeven in X-rays.