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