Near-infrared survey of high mass X-ray binary candidates

¹ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, 03080 Alicante, Spain e-mail: jmt@ua.es
² Massachusetts Institute of Technology, Kavli Institute for Astrophysics and Space Research, Cambridge MA 02139, UK
³ Physics Department and Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA
⁴ Astronomy Department, New Mexico State University, Box 30001/Department 4500, Las Cruces, NM 88003, USA

Received: 2 June 2009
Accepted: 18 October 2009

Abstract

Context. The INTEGRAL satellite is detecting a large population of new X-ray sources that were missed by previous missions because of high obscuration and, in some cases, very short duty cycles. The nature of these sources must be addressed by characterizing their optical and/or infrared counterparts.

Aims. We investigate the nature of the optical counterparts to five of these newly discovered X-ray sources.

Methods. We combine infrared spectra in the I, J,H, and K bands with JHK photometry to characterize the spectral type, luminosity class, and distance to the infrared counterparts to these systems. For IGR J19140+0951, we present spectroscopy from the red to the K band and new red and infrared photometry. For SAX J18186-1703 and IGR J18483-0311, we present the first intermediate-resolution spectroscopy to be published. Finally, for IGR J18027-2016, we present new I and K band spectra.

Results. We find that four systems harbour early-type B supergiants. All of them are heavily obscured, with ranging between 3 and 5, implying visual extinctions of ~ 9 to 15 mag. We refine the published classifications of IGR J18027-2016 and IGR J19140+0951 by constraining their luminosity class. In the first case, we confirm the supergiant nature but exclude a class III. In the second case, we propose a slightly higher luminosity class (Ia instead of Iab) and provide an improved value of the distance based on new optical photometry. Two other systems, SAX J18186-1703 and IGR J18483-0311 are classified as supergiant fast X-ray transients (SFXTs). XTE J1901+014, on the other hand, contains no bright infrared source in its error circle.

Conclusions. Owing to their infrared and X-ray characteristics, IGR J18027-2016 and IGR J19140+0951, emerge as supergiant X-ray binaries with X-ray luminosities of the order of  erg s^-1, while SAX J1818.6-1703 and IGR J18483-0311, are found to be SFXTs at 2 and 3 kpc, respectively. Finally, XTE J1901+014 emerges as a puzzling source: its X-ray behaviour is strongly reminiscent of SFXTs, but a supergiant nature is firmly excluded for the counterpart. We discuss several alternative scenarios to explain its behaviour.