A search for counterparts to massive X-ray binaries using photometric catalogues

Abstract

Context. The X-ray and γ-ray observatory INTEGRAL has discovered large numbers of new hard X-ray sources, many of which are believed to be high mass X-ray binaries. However, for a significant fraction, their counterparts remain unidentified. Aims. We explore the use of photometric catalogues to find optical counterparts to high mass X-ray binaries and search for objects likely to be early-type stars within the error circles of several INTEGRAL sources. Methods. Candidates were selected from 2MASS photometry by means of a reddening free Q parameter. Sufficiently bright candidates were spectroscopically observed. Results. Many of the candidates selected turned out to be moderately reddened late A or early F stars. Optically visible OB stars are very scarce even in these Galactic Plane fields. Our method is able to identify the counterpart to IGR J16207-5129, confirmed by a Chandra localisation. We classify this object as a BO supergiant. In the field of AX J1820.5-1434, we find a mid or early B-type star, but we cannot confirm it as the counterpart. For IGR J16320-4751 we rule out the optically visible candidate as a possible counterpart. For AX J1700.2-4220, we do not find any suitable candidate within the ASCA error circle. We classify HD 153295, a marginal candidate to be the counterpart, as B0.5 IVe, and find a distance compatible with membership in Sco OB1. In the case of IGR J17091-3624, the object associated with a variable radio source in the field is a late F star. We derive a spectral type B0 IIIe for HD 100199, previously identified as the counterpart to IGR J11305-6256. Conclusions. The procedure used is able to correctly identify OB stars and, in about one third of the cases, may lead to the localisation of the correct counterpart. However, the majority of INTEGRAL error circles do not contain any suitable optically visible counterpart. Deep infrared searches are going to be necessary in order to locate the counterparts to these sources. © ESO 2006.