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

¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: [lexk;ameer]@science.uva.nl
² Instituto de Estructura de la Materia, CSIC, Serrano 121, 29006 Madrid, Spain e-mail: najarro@damir.iem.csic.es

Received: 7 April 2006
Accepted: 28 June 2006

Abstract

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 , an effective temperature of 18 100 K and a luminosity of . 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 yr^-1 and 305 km s^-1, respectively. The interstellar 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 kpc. The luminosity derived from the model atmosphere is consistent with this distance (3 kpc). The luminosity of the wind-fed X-ray pulsar ( 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 km s^-1 yielding a mass ratio . The absence of an X-ray eclipse results in a lower limit to the mass of Wray 977 of 39 . An upper limit of 68 or 53 is derived for the mass of Wray 977 adopting a maximum neutron star mass of 3.2 or 2.5 , respectively. The corresponding lower limit to the system inclination is , 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 , consistent with the range in mass derived from the binarity constraints. The mass of the neutron star is . Time series of spectral lines formed in the dense stellar wind (e.g. 5876 Å and ) indicate the presence of a gas stream trailing the neutron star in its orbit. The long-term behaviour of the 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.