THE MASS OF THE NEUTRON-STAR IN CYGNUS X-2 (V1341 CYGNI)

Cygnus X-2 is one of the brightest and longest known X-ray sources. We present high-resolution optical spectroscopy of Cyg X-2 obtained over 4 yr, which gives an improved mass function of 0.69 +/- 0.03 M. (1 si gma). In addition, we resolve the rotationally broadened absorption fe atures of the secondary star for the first time, deriving a rotation s peed of upsilon sin i = 34.2 +/- 2.5 km s(-1) (1 sigma), which leads t o a mass ratio of q = M-c/M-X = 0.34 +/- 0.04 (1 sigma, assuming a tid ally locked and Roche lobe-filling secondary). Hence, with the lack of X-ray eclipses (i.e., less than or similar to 73 degrees) we can set firm 95% confidence lower limits to the neutron star mass of M-X > 1.2 7 M. and to the companion star mass of M-c > 0.39 M.. However, by addi tionally requiring that the companion must exceed 0.75 M. (as required theoretically to produce a steady low-mass X-ray binary), then M-X > 1.88 M. and i < 61 degrees (95% confidence lower and upper limit, resp ectively), thereby making Cyg X-2 the highest mass neutron star measur ed to date. If confirmed, this would set significant constraints on th e equation of state of nuclear matter.