Swift follow-up observations of 17 INTEGRAL sources of uncertain or unknown nature

¹ Laboratoire AIM, CEA/DSM - CNRS - Université Paris Diderot, IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette, France e-mail: jrodriguez@cea.fr
² Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA

Received: 8 August 2008
Accepted: 25 November 2008

Abstract

Context. The positional accuracy of the IBIS telescope on-board INTEGRAL, albeit unprecedented in the >20 keV range, is still not good enough to identify many hard X-ray sources discovered by INTEGRAL. This indeed prevents counterparts from being found at other wavelengths, which is the only way to unveil the true nature of these sources.

Aims. We continue the work of trying to reveal the nature of these hard X-ray sources. This is done by analysing X-ray data collected via focusing X-ray telescopes, with the primary goal of discovering soft X-ray counterparts of the INTEGRAL sources to provide an accurate X-ray position. With a few arcsec accuracy, we can identify counterparts at infrared and optical wavelengths.

Methods. We analysed data from observations of 17 INTEGRAL sources made with the Swift satellite. The X-ray images obtained by the X-ray Telescope instrument allowed us to refine the position of the hard X-ray sources to an accuracy of a few arcsec. We then browsed the online catalogues (e.g., NED, SIMBAD, 2MASS, 2MASX, USNO) to search for counterparts at other wavelengths. We also made use of the X-ray spectral parameters to further distinguish between the various possibilities.

Results. For 13 sources, we find the X-ray counterpart without any ambiguity. For these, we provide the position with arcsec accuracy, identify possible infrared and optical counterparts (when found), give the magnitudes in those bands and in the optical and UV as seen with the Swift UVOT telescope when observations are available. We confirm the previously suggested associations and source types for IGR J03532-6829, J05346-5759, J10101-5654, J13000+2529, J13020-6359, J15479-4529, J18214-1318, and J23206+6431. We identify IGR J09025-6814 as an AGN for the first time, and we suggest that it may be a Seyfert 2. We suggest that IGR J05319-6601, J16287-5021, J17353-3539, and J17476-2253 are X-ray binaries, with J05319-6601 located in the LMC and the other three possibly being HMXBs in our Galaxy. For IGR J15161-3827 and J20286+2544, we find several possible X-ray counterparts in the IBIS error region, and we discuss which, if any, are the likely counterparts. Both are likely AGNs, although the latter could be a blend of two AGNs. For IGR J03184-0014 and J19267+1325, we find X-ray sources slightly outside the IBIS error circle. In the former, we do not favour an association of the Swift and INTEGRAL source, while it is very likely that IGR J19267+1325 and the Swift source are the same.