Multi-wavelength observations of Galactic hard X-ray sources discovered by INTEGRAL^*,**

I. The nature of the companion star

¹ Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, Irfu/Service d'Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France e-mail: chaty@cea.fr
² ESO, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
³ Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
⁴ Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA
⁵ INTEGRAL Science Data Centre, Chemin d'Écogia 16, 1290 Versoix, Switzerland

Received: 30 September 2007
Accepted: 15 December 2007

Abstract

Context. The INTEGRAL hard X-ray observatory has revealed an emerging population of highly obscured X-ray binary systems through multi-wavelength observations. Previous studies have shown that many of these sources are high-mass X-ray binaries hosting neutron stars orbiting around luminous and evolved companion stars.

Aims. To better understand this newly-discovered population, we have selected a sample of sources for which an accurate localisation is available to identify the stellar counterpart and reveal the nature of the companion star and of the binary system.

Methods. We performed an intensive study of a sample of thirteen INTEGRAL sources, through multi-wavelength optical to NIR photometric and spectroscopic observations, using EMMI and SofI instruments at the ESO NTT telescope. We performed accurate astrometry and identified candidate counterparts for which we give the optical and NIR magnitudes. We detected many spectral lines allowing us to determine the spectral type of the companion star. We fitted with stellar black bodies the mid-infrared to optical spectral energy distributions of these sources. From the spectral analysis and SED fitting we identified the nature of the companion stars and of the binary systems.

Results. Through spectroscopic analysis of the most likely candidates we found the spectral types of IGR J16320-4751, IGR J16358-4726, IGR J16479-4514, IGR J17252-3616, IGR J18027-2016: They all host OB type supergiant companion stars, with IGR J16358-4726 likely hosting an sgB[e]. Our spectra also confirm the supergiant O and B nature of IGR J17391-3021 and IGR J19140+0951. From SED fitting we found that IGR J16418-4532 is a (likely OB supergiant) HMXB, IGR J16393-4643 a (likely BIV-V star) HMXB, and IGR J18483-0311 a likely HMXB system. Through accurate astrometry, we rejected the proposed counterparts of IGR J17091-3624 and IGR J17597-2201, and we discovered two new candidate counterparts for each source, both suggesting an LMXB from SED fitting. We confirm the AGN nature of IGR J16558-5203. Finally, we report that NIR fields of four sources of our sample exhibit large-scale regions of absorption.

Conclusions. The majority of these systems are high-mass X-ray binaries hosting supergiant companion stars. We therefore confirm that INTEGRAL reveals a dominant class of obscured and short-lived high-energy binary systems, and we suggest an association of these systems with regions of the Galaxy exhibiting large-scale absorption. Stellar population models should take these systems into account for realistic estimates of high-energy binary systems in our Galaxy.