On the mass of the neutron star in Cyg X-2

Abstract

We present new high resolution spectroscopy of the low mass X-ray binary Cyg X-2 which enables us to refine the orbital solution and rotational broadening of the donor star. In contrast with Elebert et al (2009) we find a good agreement with results reported in Casares et al. (1998). We measure $P=9.84450\pm 0.00019$ day, $K_2=86.5\pm 1.2$ km s${}^{-1}$ and $V\sin i=33.7\pm 0.9$ km s${}^{-1}$. These values imply $q=M_2/M_1=0.34\pm 0.02$ and $M_1=1.71\pm 0.21$ M${}_{\odot }$ (for $i=62.5\pm 4^{\circ }$). Therefore, the neutron star in Cyg X-2 can be more massive than canonical. We also find no evidence for irradiation effects in our radial velocity curve which could explain the discrepancy between Elebert et al's and our $K_2$ values.

Accepted for publication in MNRAS

References in corpus (3)

• The "K-Correction" for Irradiated Emission Lines in LMXBs: Evidence for a Massive Neutron Star in X1822-371 (V691 CrA)
• A detection of the donor star of Aquila X-1 during its 2004 outburst?
• Optical spectroscopy and Doppler tomography of Cygnus X-2

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