IngentaConnect The radial velocity of the companion star in the low-mass X-ray b...

Authors: P. G. Jonker; D. Steeghs¹; G. Nelemans²; M. van der Klis³

Source: Monthly Notices of the Royal Astronomical Society, Volume 356, Number 2, January 2004 , pp. 621-626(6)

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 M_X sin³i > 1.90 ± 0.25 M (1 sigma

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 < M_X < 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.

Keywords: accretion: accretion discs; binaries: general; stars: individual: 2S 0921–630; stars: neutron; X-rays: binaries

Document Type: Research article

DOI: 10.1111/j.1365-2966.2004.08476.x

Affiliations: 1: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 2: Institute of Astronomy, Madingley Road, Cambridge CB3 0HA 3: Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, the Netherlands

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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