VLT/UVES spectroscopy of Wray 977, the hypergiant companion to the X-ray pulsar GX301-2

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 62 R_o, an effective temperature of 18 100 K and a luminosity of 5 × 10⁵ L_o. 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^-5 M_o yr^-1 and 305 km s^-1, respectively. The interstellar Na I D absorption indicates that Wray 977 is located behind the first intersection with the Sagittarius-Carina spiral arm (1-2.5 kpc) and probably belongs to the stellar population of the Norma spiral arm at a distance of 3-4 kpc. The luminosity derived from the model atmosphere is consistent with this distance (3 kpc). The luminosity of the wind-fed X-ray pulsar (L_X ~ 10³⁷ erg s^-1) 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 ± 3 km s^-1 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 39 M_o. An upper limit of 68 or 53 M_o is derived for the mass of Wray 977 adopting a maximum neutron star mass of 3.2 or 2.5 M_o, 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 2002). The "spectroscopic" mass of Wray 977 is 43 ± 10 M_o, consistent with the range in mass derived from the binarity constraints. The mass of the neutron star is 1.85 ± 0.6 M_o. Time series of spectral lines formed in the dense stellar wind (e.g. He I 5876 Å and Hα) indicate the presence of a gas stream trailing the neutron star in its orbit. The long-term behaviour of the Hα 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.