2006A&A...457..595K

Model atmosphere fits to high-resolution optical spectra of Wray 977 confirm the B hypergiant classification of the massive companion to the X-ray pulsar GX301-2. The models give a radius of 62R_☉, an effective temperature of 18100K and a luminosity of 5x10⁵L_☉. These values are somewhat reduced compared to the stellar parameters of Wray 977 measured previously. The deduced mass-loss rate and terminal velocity of the stellar wind are 10^–5M_☉/yr and 305km/s, respectively. The interstellar NaI D absorption indicates that Wray 977 is located behind the first intersection with the Sagittarius-Carina spiral arm (1-2.5kpc) and probably belongs to the stellar population of the Norma spiral arm at a distance of 3-4kpc. The luminosity derived from the model atmosphere is consistent with this distance (3kpc). The luminosity of the wind-fed X-ray pulsar (L_X∼10³⁷erg/s) is in good accordance with the Bondi-Hoyle mass accretion rate. The spectra obtained with UVES on the Very Large Telescope (VLT) cover a full orbit of the system, including periastron passage, from which we derive the radial-velocity curve of the B hypergiant. The measured radial-velocity amplitude is 10±3km/s yielding a mass ratio q=M_X/M_opt=0.046±0.014. The absence of an X-ray eclipse results in a lower limit to the mass of Wray 977 of 39M_☉. An upper limit of 68 or 53M_☉ is derived for the mass of Wray 977 adopting a maximum neutron star mass of 3.2 or 2.5M_☉, respectively. The corresponding lower limit to the system inclination is i>44°, supporting the view that the dip in the X-ray lightcurve is due to absorption by the dense stellar wind of Wray 977 (Leahy, 2002A&A...391..219L). The ``spectroscopic'' mass of Wray 977 is 43±10M_☉, consistent with the range in mass derived from the binarity constraints. The mass of the neutron star is 1.85±0.6M_☉. Time series of spectral lines formed in the dense stellar wind (e.g. HeI 5876Å and H α) indicate the presence of a gas stream trailing the neutron star in its orbit. The long-term behaviour of the H α equivalent width exhibits strong variations in wind strength; the sampling of the data is insufficient to conclude whether a relation exists between wind mass-loss rate and pulsar spin period.