WRAY-977 (GX301-2) - A HYPERGIANT WITH PULSAR COMPANION

Wray 977, the B supergiant companion of the X-ray pulsar GX301-2, should be classified as B 1 Ia+, based on a comparison of its optical spectrum to that of zeta(1) 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 = 5.3 kpc (previously 1.8 +/- 0.3 kpc). The ''average'' X-ray luminosity of the pulsar is then similar to 10(37) erg s(-1), in good agreement with the predicted X-ray luminosity resulting from accretion of a dense, low-velocity (v(infinity) = 400 km s(-1)) stellar wind. A mass-loss rate of less than or similar to 10(-5) M. yr(-1) is estimated from the H alpha 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) greater than or similar to 50 M..