Jonker, P. G., Steeghs, D., Nelemans, G. and van der Klis, M. (2005) 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. Monthly Notices of the Royal Astronomical Society, Vol.356 (No.2). pp. 621-626. doi:10.1111/j.1365-2966.2004.08476.x ISSN 0035-8711.

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Abstract

In this paper we report on optical spectroscopic observations of the low-mass X-ray binary 2S 0921–630 obtained with the Very Large Telescope. We found sinusoidal radial velocity variations of the companion star with a semi-amplitude of 99.1 ± 3.1 km s−1 modulated on a period of 9.006 ± 0.007 d, consistent with the orbital period found previously for this source, and a systemic velocity of 44.4 ± 2.4 km s−1. Owing to X-ray irradiation, the centre of light measured by the absorption lines from the companion star is probably shifted with respect to the centre of mass. We try to correct for this using the so-called K-correction. Conservatively applying the maximum correction possible and using the previously measured rotational velocity of the companion star, we find a lower limit to the mass of the compact object in 2S 0921–630 of MX sin3i > 1.90 ± 0.25 M⊙ (1σ errors). The inclination in this system is well constrained since partial eclipses have been observed in X-ray and optical bands. For inclinations in the range 60° < i < 90° we find 1.90 ± 0.25 < MX < 2.9 ± 0.4 M⊙. However, using this maximum K-correction we find that the ratio between the mass of the companion star and that of the compact object, q, is 1.32 ± 0.37, implying super-Eddington mass-transfer rates; however, evidence for that has not been found in 2S 0921–630. We conclude that the compact object in 2S 0921–630 is either a (massive) neutron star or a low-mass black hole.

Item Type:	Journal Article
Subjects:	Q Science > QB Astronomy
Divisions:	Faculty of Science, Engineering and Medicine > Science > Physics
Library of Congress Subject Headings (LCSH):	X-ray binaries
Journal or Publication Title:	Monthly Notices of the Royal Astronomical Society
Publisher:	Wiley
ISSN:	0035-8711
Official Date:	2005
Dates:	Date Event 2005 Published
Volume:	Vol.356
Number:	No.2
Page Range:	pp. 621-626
DOI:	10.1111/j.1365-2966.2004.08476.x
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
Funder:	United States. National Aeronautics and Space Administration (NASA), Smithsonian Astrophysical Observatory, Particle Physics and Astronomy Research Council (Great Britain) (PPARC)
Grant number:	PF3–40027 (NASA)

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