2011A&A...533A..23R

Swift J1626.6-5156 is an X-ray pulsar that was discovered in December 2005 during an X-ray outburst. Although the X-ray data suggest that the system is a high-mass X-ray binary, very little information exists on the nature of the optical counterpart. We investigate the emission properties of the optical counterpart in the optical and near-IR bands and the long-term X-ray variability of the system to determine unambiguously the nature of this X-ray pulsar. We have performed an X/optical/IR analysis of Swift J1626.6-5156. We have analysed all RXTE observations since its discovery, archived optical spectroscopic and photometric data and obtained near-IR spectra for the first time. X-ray energy spectra were fitted with models composed of a combination of photoelectric absorption, a power law with high-energy exponential cutoff and a Gaussian line profile at 6.5keV and an absorption edge at around 9keV. X-ray power spectra were fitted with Lorentzian profiles. We identified and measured the equivalent width and relative intensity of the spectral features in the optical and infrared spectra to determine the spectral type of the optical counterpart. The K-band spectrum shows HeI λ20581Å and HI λ21660Å (Brackett-gamma) in emission, which confines the spectral type of the companion to be earlier than B2.5. The H-band spectrum exhibits the HI Br-18-11 recombination series in emission. The most prominent feature of the optical band spectrum is the strong emission of the Balmer line Hα. The 4000-5000Å spectrum contains HeII and numerous HeI lines in absorption, indicating an early B-type star. The source shows three consecutive stages characterised by different types of variability in the X-ray band: a smooth decay after the peak of a major outburst, high-amplitude flaring variability (reminiscent of type I outbursts) and quiescence. We observed that the spectrum becomes softer as the flux decreases and that this is a common characteristic of the X-ray emission for all observing epochs. An emission line feature at ∼6.5keV is also always present. The X-ray/optical/IR continuum and spectral features are typical of an accreting X-ray pulsar with an early-type donor. The long-term X-ray variability with its major and minor outbursts and quiescent emission is also characteristic of hard X-ray transients. We conclude that Swift J1626.6-5156 is a Be/X-ray binary with a B0Ve companion located at a distance of ∼10kpc.