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The mass of the neutron star in Cyg X-2 (V1341 Cyg) | | Jorge Casares
; Phil Charles
; Erik Kuulkers
; | | Date: |
20 Nov 1997 | | Subject: | astro-ph | | Affiliation: | IAC), Phil Charles (Oxford University), Erik Kuulkers (Oxford University | | 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 years which gives an improved mass function of 0.69 +/- 0.03 Msun (1 sigma error). In addition, we resolve the rotationally broadened absorption features of the secondary star for the first time, deriving a rotation speed of vsin(i) = 34.2 +/- 2.5 km per s (1 sigma error) which leads to a mass ratio of q = M_c/M_x = 0.34 +/- 0.04 (1 sigma error), assuming a tidally-locked and Roche lobe-filling secondary). Hence with the lack of X-ray eclipses (i.e. i <~ 73 degrees) we can set firm 95% confidence lower limits to the neutron star mass of M_x > 1.27 Msun and to the companion star mass of M_c > 0.39 Msun. However, by additionally requiring that the companion must exceed 0.75 Msun (as required theoretically to produce a steady low-mass X-ray binary), then M_x > 1.88 Msun and i < 61 degrees (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. | | Source: | arXiv, astro-ph/9711236 | | Services: | Forum | Review | PDF | Favorites | |
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