The Mass of the Neutron Star in Cygnus X-2 (V1341 Cygni)

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Abstract
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 v sin i=34.2±2.5 km s-1 (1 σ), which leads to a mass ratio of q=Mc/M X = 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., i73°) 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.
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