Determination of the mass of the neutron star in SMC X-1, LMC X-4, and Cen X-3 with VLT/UVES

Abstract

We present the results of a spectroscopic monitoring campaign of the OB-star companions to the eclipsing X-ray pulsars SMC X−1, LMC X−4 and Cen X−3. High-resolution optical spectra obtained with UVES on the ESO Very Large Telescope are used to determine the radial-velocity orbit of the OB (super)giants with high precision. The excellent quality of the spectra provides the opportunity to measure the radial-velocity curve based on individual lines, and to study the effect of possible distortions of the line profiles due to e.g. X-ray heating on the derived radial-velocity amplitude. Several spectral lines show intrinsic variations with orbital phase. The magnitude of these variations depends on line strength, and thus provides a criterion to select lines that do not suffer from distortions. The undistorted lines show a larger radial-velocity amplitude than the distorted lines, consistent with model predictions. Application of our line-selection criteria results in a mean radial-velocity amplitude K opt of 20.2± 1.1, 35.1± 1.5, and 27.5± 2.3 km s −1 (1σ errors), for the OB companion to SMC X−1, LMC X−4 and Cen X−3, respectively. Adding information on the projected rotational velocity of the OB companion (derived from our spectra), the duration of X-ray eclipse and orbital parameters of the X-ray pulsar (obtained from literature), we arrive at a neutron star mass of 1.06−0.10 , 1.25−0.10 and 1.34 +0.16−0.14 M for SMC X−1, LMC X−4 and Cen X−3, respectively. The mass of SMC X−1 is near the minimum mass (∼1 M ) expected for a neutron star produced in a supernova. We discuss the implications of the measured mass distribution on the neutron-star formation mechanism, in relation to the evolutionary history of the massive binaries.

Spin period evolution of a recycled pulsar in an accreting binary

“…First, with a small amount of mass transferred, the NS is spun up from the death valley with a long period and strong field, e.g. the NS in HMXBs (B ∼ 10 12 G, P ∼ 10 s) like Her X-1 (P = 1.24 s, B = 3 × 10 12 G, see van der Meer et al 2007;Liu et al 2007). With the long-lived accretion phase, the binary accepts sufficient mass from its companion and the NS yields a substantial field decay, as in the case of MSPs (Bhattacharya & van den Heuvel 1991), e.g., the fastest known pulsar PSR J1748-2446 (B ∼ 10 8 G, P = 1.4 ms, see Hessels et al 2006).…”

“…When a small amount of mass (∼0.001 M ) is transferred, the spin period changes mildly (Fig. 6), which may yield a HMXB with the spin period of some seconds, such as Her X-1 (van der Meer et al 2007) and Vela X-1 (Quaintrell et al 2003). After a conveniently long time, with the weight of material accreted ∼0.01 M , NS is spun up to a shorter period (P ∼ 100 ms), which corresponds to the system of DNS, e.g.…”

Tidal interaction in High‐Mass X‐ray Binaries

“…Cen X-3 (V779 Cen) -the stellar parameters are taken from Ash et al (1999) and van der Meer et al (2007). We calculate P ps /P rot ≈ 0.72 ± 0.1.…”

“…This long-term variation is attributed to the precessing accretion disc. The stellar parameters are taken from van der Meer et al (2007). P ps /P rot ≈ 0.92±0.04.…”

“…The stellar parameters are taken from van der Meer et al (2007). SMC X-1 demonstrates an orbital variation of 3.89 days and a superorbital variation with an average length of ∼ 55 days (Trowbridge, Nowak & Wilms 2007).…”