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

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 R , an effective temperature of 18 100 K and a luminosity of 5 × 10 5 L . 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 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 ∼ 10 37 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 . An upper limit of 68 or 53 M is derived for the mass of Wray 977 adopting a maximum neutron star mass of 3.2 or 2.5 M , 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 , consistent with the range in mass derived from the binarity constraints. The mass of the neutron star is 1.85 ± 0.6 M . 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.

A VLT/FLAMES survey for massive binaries in Westerlund 1

“…3.2, the anomalous C-abundance of Wd1-5 (C/C ∼ 1.4) is also present in the B1 Ia + and O6.5 Iaf + hypergiant mass donors in the X-ray binaries GX301-2 and 4U1700-37. As with our putative Wd1-5 + J1647-45 binary, the current physical properties of both binaries imply pre-SN mass transfer onto the current mass donor (Wellstein & Langer 1999;Clark et al 2002;Kaper et al 2006). Additionally, the lifetime of the C-abundance anomaly is expected to be rather short.…”

“…In addition to the expected He-and N-enrichment, both show significant C-enrichment over CNO equilibrium values (C/C ∼ 2 and 1 respectively; Kaper et al 2006;Clark et al 2002) in an analagous manner to Wd1-5; the implications of this finding are discussed below.…”

“…Illustrative spectra of selected, closely related early-B hypergiants and WNVLh stars are also shown (with BP Cru and Wd1-13 overplotted in dashed cyan and green lines, respectively, to save space). Spectra of Wd1-13 and 44 were not available in the 5600−6000 Å window, while data on ζ 1 Sco and BP Cru were from Clark et al (2012) and Kaper et al (2006). Both Wd1-13 and -44 are SB2 binaries, although a precise spectral classification of their secondaries is uncertain at this time.…”

“…Appendix A: Summary of stellar comparison data to Wd1-5 Crowther et al (2006b); (2) Lefever et al (2007); (3) Searle et al (2008); (4) Markova & Puls (2008); (5) Kaper et al (2006); (6) Clark et al (2012); (7) Groh et al (2009a); (8) Najarro (2001); (9) Groh et al (2009b); (10) Najarro et al (2009); (11) Clark et al (2009b); (12) Bohannan & Crowther (1999); (13) Martins et al (2007). …”

Tidal interaction in High‐Mass X‐ray Binaries

“…The system parameters are from Kaper, van der Meer & Najarro (2006). We calculate P ps /P rot ≈ 0.31 ± 0.03.…”