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 σ). In addition, we resolve the rotationally broadened absorption features of the secondary star for the first time, deriving a rotation speed of ν sin i = 34.2 ± 2.5 km s-1 (1 σ), which leads to a mass ratio of q = Mc/Mx = 0.34 ± 0.04 (1 σ, assuming a tidally locked and Roche lobe-filling secondary). Hence, with the lack of X-ray eclipses (i.e., i ≲ 73°) we can set firm 95% confidence lower limits to the neutron star mass of Mx > 1.27 M⊙ and to the companion star mass of Mc > 0.39 M⊙. However, by additionally requiring that the companion must exceed 0.75 M⊙ (as required theoretically to produce a steady low-mass X-ray binary), then Mx > 1.88 M⊙ and i < 61° (95% confidence lower and upper limit, respectively), thereby making Cyg X-2 the highest mass neutron star measured to date. If confirmed, this would set significant constraints on the equation of state of nuclear matter. ©. The American Astronomical Society. All rights reserved.