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

TL;DR

This study characterizes the donor star in the X-ray binary X1908+075 using infrared spectra and stellar atmosphere modeling, revealing its type, parameters, and a refined distance estimate, thus enhancing understanding of such obscured systems.

Contribution

First stellar atmosphere model for the donor star in X1908+075, providing detailed stellar parameters and a more accurate distance measurement.

Findings

Donor star is an early B-type supergiant.

Estimated stellar mass is 15 ± 6 solar masses.

Distance to the system is refined to 4.85 ± 0.50 kpc.

Abstract

High-mass X-ray binaries (HMXBs) 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: $M_{\mathrm{spec}}$ = $15 \pm 6$$M_{\odot}$, $T_{\ast} = 23_{-3}^{+6},$kK, $\log g_\mathrm{eff} = 3.0 \pm 0.2$ and $\log L/L_{\odot}$ = $4.81 \pm 0.25$. 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 $\pm$ 0.50 kpc than the previously reported value.