The donor star of the X-ray pulsar X1908+075

High-mass X-ray binaries consist of a massive donor star and a compact object. While several of those systems have been well studied in X-rays, little is known for most of the donor stars as they are often heavily obscured in the optical and ultraviolet regime. There is an opportunity to observe them at infrared wavelengths, however. The goal of this study is to obtain the stellar and wind parameters of the donor star in the X1908+075 high-mass X-ray binary system with a stellar atmosphere model to check whether previous studies from X-ray observations and spectral morphology lead to a sufficient description of the donor star. We obtained H- and K-band spectra of X1908+075 and analysed them with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. For the first time, we calculated a stellar atmosphere model for the donor star, whose main parameters are: Mspec = 15 ± 6 M⊙, T∗ = 23-3+6 kK, log geff = 3.0 ± 0.2 and log L/L⊙ = 4.81 ± 0.25. The obtained parameters point towards an early B-type (B0–B3) star, probably in a supergiant phase. Moreover we determined a more accurate distance to the system of 4.85 ± 0.50 kpc than the previously reported value.

This work was supported by the Spanish Ministerio de Ciencia e Innovación through the project AYA2010-15431. S.M.N. acknowledges the support of the Spanish Unemployment Agency, which allowed her to continue her scientific collaborations during the critical situation of the Spanish Research System. A.S. is supported by the Deutsche Forschungsgemeinschaft (DFG) under grant HA 1455/22. A.G.G. acknowledges support by Spanish MICINN under FPI Fellowship BES-2011-050874 and the Vicerectorat d’Investigació, Desenvolupament i Innovació de la Universitat d’Alacant under project GRE12-35. A.G.G. is supported by the Deutsches Zentrum für Luft und Raumfahrt (DLR) under contract No. FKZ 50 OR 1404.