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 years which gives an improved mass function of 0.69 \pm 0.03 M _ { \odot } ( 1 \sigma ) .
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 \pm 2.5 km s ^ { -1 } ( 1 \sigma ) which leads to a mass ratio of q = M _ { c } / M _ { x } = 0.34 \pm 0.04 ( 1 \sigma , assuming a tidally-locked and Roche lobe-filling secondary ) .
Hence with the lack of X-ray eclipses ( i.e . i \lesssim 73 ^ { \circ } ) we can set firm 95 % confidence lower limits to the neutron star mass of M _ { x } > 1.27 M _ { \odot } and to the companion star mass of M _ { c } > 0.39 M _ { \odot } .
However , by additionally requiring that the companion must exceed 0.75 M _ { \odot } ( as required theoretically to produce a steady low-mass X-ray binary ) , then M _ { x } > 1.88 M _ { \odot } and i < 61 ^ { \circ } ( 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 .