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

¹ X-ray Astronomy Group, Departamento de Física, Ingeniería de Sistemas y Teoría de la SeñalUniversity of Alicante, PO Box 99, 03080 Alicante, Spain
e-mail: silvia.martinez.nunez@gmail.com
² Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
³ Instituto Universitario de Física Aplicada a las Ciencias y las Tecnologías, University of Alicante, PO Box 99, 03080 Alicante, Spain
⁴ Institute of Astronomy, University of Cambridge, Madingly Road, Cambridge, CB3 0HA, UK

Received: 17 August 2014
Accepted: 21 April 2015

Abstract

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: M_spec = 15 ± 6 M_⊙, T_∗ = 23_-3⁺⁶ kK, log g_eff = 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.