Tidal effects on the radial velocity curve of HD77581 (Vela X-1)

Abstract

The mass of the neutron star in Vela X-1 has been found to be more massive than the canonical 1.5 Mo. This result relies on the assumption that the amplitude of the optical component's measured radial velocity curve is not seriously affected by the interactions in the system. In this paper we explore the effect on the radial velocity curve caused by surface motions excited by tidal interactions. We use a calculation from first principles that involves solving the equations of motion of a Lagrangian grid of surface elements. The velocities on the visible surface of the star are projected along the line-of-sight to the observer to obtain the absorption-line profile in the observer's reference frame. The centroid of the line-profiles for different orbital phases is then measured and a simulated RV curve constructed. Models are run for the "standard" (vsini=116 km/s) and "slow" (56 km/s) supergiant rotation velocities. We find that the surface velocity field is complex and includes fast, small-spatial scale structures. It leads to strong variability in the photospheric line profiles which, in turn, causes significant deviations from a Keplerian RV curve. The peak-to-peak amplitudes of model RV curves are in all cases larger than the amplitude of the orbital motion. Keplerian fits to RV curves obtained with the "standard" rotation velocity imply a neutron star >1.7 Mo. However, a similar analysis of the "slow" rotational velocity models allows for m_ns ~ 1.5 Mo. Thus, the stellar rotation plays an important role in determining the characteristics of the perturbed RV curve. Given the observational uncertainty in GP Vel's projected rotation velocity and the strong perturbations seen in the published and the model RV curves, we are unable to rule out a small (~1.5 Mo) mass for the neutron star companion.

14 pages, 16 figures; A&A, accepted

References in corpus (9)

• Shapiro delay measurement of a two solar mass neutron star
• Neutron Star Observations: Prognosis for Equation of State Constraints
• Refined Neutron-Star Mass Determinations for Six Eclipsing X-Ray Pulsar Binaries
• Dense Matter in Compact Stars: Theoretical Developments and Observational Constraints
• High variability in Vela X-1: giant flares and off states
• Determination of the mass of the neutron star in SMC X-1, LMC X-4 and Cen X-3 with VLT/UVES
• Eccentric binaries: Tidal flows and periastron events
• Tides in asynchronous binary systems
• The X-ray binary 2S0114+650=LSI+65 010:A slow pulsar or tidally-induced pulsations?

Cited by in corpus (13)

• On the Mass Distribution and Birth Masses of Neutron Stars
• High-Mass X-ray Binaries in the Milky Way: A closer look with INTEGRAL
• The ephemeris, orbital decay, and masses of 10 eclipsing HMXBs
• The Ups & Downs of Accreting X-Ray Pulsars: Decade-long observations with the Fermi Gamma-Ray Burst Monitor
• Measuring the stellar wind parameters in IGR J17544-2619 and Vela X-1 constrains the accretion physics in Supergiant Fast X-ray Transient and classical Supergiant X-ray Binaries
• Revisiting the archetypical wind accretor Vela X-1 in depth -- A case study of a well-known X-ray binary and the limits of our knowledge
• Exploring the origin of magnetic fields in massive stars: II. New magnetic field measurements in cluster and field stars
• Modelling interstellar structures around Vela X-1
• Astrophysical parameters of the peculiar X-ray transient IGR J11215-5952
• Spectral evidence of an accretion disk in wind-fed X-ray pulsar Vela X-1 during an unusual spin-up period
• Spectral modelling of the Alpha Virginis (Spica) binary system
• Induced differential rotation and mixing in asynchronous binary stars
• The close environment of high-mass X-ray binaries at high angular resolution I. VLTI/AMBER and VLTI/PIONIER near-infrared interferometric observations of Vela X-1