On the mass of the neutron star in V395 Car/2S 0921-630

Abstract

We report high-resolution optical spectroscopy of the low-mass X-ray binary V395 Car/2S 0921-630 obtained with the MIKE echelle spectrograph on the Magellan-Clay telescope. Our spectra are obtained near superior conjunction of the mass donor star and we exploit the absorption lines originating from the back-side of the K-type object to accurately derive its rotational velocity. Using K0-K1 III templates, we find vsini=32.9 +/- 0.8 km/s. We show that the choice of template star and the assumed limb darkening coefficient has little impact on the derived rotational velocity. This value is a significant revision downwards compared to previously published values. We derive new system parameter constraints in the light of our much lower rotational velocity. We find M_1=1.44 +/- 0.10 Msun, M_2=0.35 +/- 0.03 Msun, and q=0.24 +/- 0.02 where the errors have been estimated through a Monte-Carlo simulation. A possible remaining systematic effect is the fact that we may be over-estimating the orbital velocity of the mass donor due to irradiation effects. However, any correction for this effect will only reduce the compact object mass further, down to a minimum mass of M_1=1.05 +/- 0.08 Msun. There is thus strong evidence that the compact object in this binary is a neutron star of rather typical mass and that the previously reported mass values of 2-4Msun were too high due to an over-estimate of the rotational broadening.

arXiv comment4 pages, 3 figures, accepted for publication in ApJ Letters

References in corpus (5)

• Neutron Star Structure and the Equation of State
• The Physics of Neutron Stars
• The "K-Correction" for Irradiated Emission Lines in LMXBs: Evidence for a Massive Neutron Star in X1822-371 (V691 CrA)
• The radial velocity of the companion star in the low-mass X-ray binary 2S 0921-630: limits on the mass of the compact object
• The massive neutron star or low-mass black hole in 2S0921-630

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