1995A&A...300..446K

Wray 977, the B supergiant companion of the X-ray pulsar GX 301-2, should be classified as B1 Ia+, based on a comparison of its optical spectrum to that of ζ1 Sco, a well-known B hypergiant and one of the brightest stars in our Galaxy. The classification of Wray 977 as a hypergiant results in a new distance determination of the binary system, i.e. d=5.3kpc (previously 1.8±0.4kpc). The "average" X-ray luminosity of the pulsar is then ~10³⁷erg/s, in good agreement with the predicted X-ray luminosity resulting from accretion of a dense, low-velocity (v_∞=400km/s) stellar wind. A mass-loss rate of ≲10^–5M☉/yr is estimated from the Hα profile. A new upper limit for the inclination of the system is derived which provides a lower limit to the (present) mass of Wray 977 (48 M☉). Regarding current binary evolution scenarios, the empirical lower mass limit for black-hole formation in a binary increases to M_ZAMS ≳ 50M☉.