The radial velocity curve of HD153919 (4U1700-37) revisited

We have re-analysed all available high-resolution ultraviolet IUE spectra of the high-mass X-ray binary HD153919/4U1700-37. The radial velocity semi-amplitude of 20.6 +/- 1.0 km/s and orbital eccentricity of 0.22 +/- 0.04 agree very well with the values obtained earlier from optical spectra. They disagree with earlier conclusions for the same data reduced by Heap & Corcoran (1992) and by Stickland & Lloyd (1993).Comment: 6 pages, latex, figure included, Astronomy & Astrophysics, in pres

Modelling the orbital modulation of ultraviolet resonance lines in high-mass X-ray binaries

The stellar-wind structure in high-mass X-ray binaries (HMXBs) is investigated through modelling of their ultraviolet (UV) resonance lines. For the OB supergiants in two systems, Vela X-1 and 4U1700-37, high-resolution UV spectra are available; for Cyg X-1, SMC X-1, and LMC X-4 low-resolution spectra are used. In order to account for the non-monotonic velocity structure of the stellar wind, a modified version of the Sobolev Exact Integration (SEI) method by Lamers et al. (1987) is applied. The orbital modulation of the UV resonance lines provides information on the size of the Stroemgren zone surrounding the X-ray source. The amplitude of the observed orbital modulation (known as the Hatchett-McCray effect), however, also depends on the density- and velocity structure of the ambient wind. Model profiles are presented that illustrate the effect on the appearance of the HM effect by varying stellar-wind parameters. The q parameter of Hatchett & McCray (1977), as well as other parameters describing the supergiant's wind structure, are derived for the 5 systems. The X-ray luminosity needed to create the observed size of the Stroemgren zone is consistent with the observed X-ray flux. The derived wind parameters are compared to those determined in single OB supergiants of similar spectral type. Our models naturally explain the observed absence of the HM effect in 4U1700-37. The orbital modulation in Vela X-1 indicates that besides the Stroemgren zone other structures are present in the stellar wind (such as a photo-ionization wake). The ratio of the wind velocity and the escape velocity is found to be lower in OB supergiants in HMXBs than in single OB supergiants of the same effective temperature.Comment: 29 pages, good quality figures 11, 12, 13, A2 & B1 available upon request from JvL. Accepted for publication in Astronomy & Astrophysic

Spectroscopy of HD 77581 and the mass of Vela X-1

We present new high-resolution, high signal-to-noise optical spectra of HD 77581, the optical counterpart of the X-ray source Vela X-1, and determine radial velocities from these spectra, as well as from high-resolution IUE spectra and from digitized photographic spectra. The measured velocities show strong deviations from a pure Keplerian radial-velocity curve, which are autocorrelated within one night, but not from one night to another. Since lines of different ions exhibit very similar changes in profile, these deviations most likely reflect large-scale motions of the stellar surface akin to non-radial pulsations. A possible cause could be that the varying tidal force exerted by the neutron star in its eccentric orbit excites high-order pulsation modes in the optical star which interfere constructively for short time intervals. The effect of such velocity excursions on the orbital solution is estimated by means of a Monte-Carlo simulation technique. We investigate sources of systematic error, due to, e.g., the tidal deformation of the star, and find, in particular, evidence for a systematic perturbation of the radial velocity near the time of velocity minimum. This possible distortion severely compromises the accuracy of the radial-velocity amplitude, leading to a 95\% confidence range of 18.0--28.2\,\kms. The corresponding 95\% confidence limits of the masses are given by \MX=1.9_{-0.5}^{+0.7}\,\Msun and \Mopt=23.5_{-1.5}^{+2.2}\,\Msun

The hard X-ray emission of X Per

We present an analysis of the spectral properties of the peculiar X-ray pulsar X Per based on INTEGRAL observations. We show that the source exhibits an unusually hard spectrum and is confidently detected by ISGRI up to more than 100 keV. We find that two distinct components may be identified in the broadband 4-200 keV spectrum of the source. We interpret these components as the result of thermal and bulk Comptonization in the vicinity of the neutron star and describe them with several semi-phenomenological models. The previously reported absorption feature at ~30 keV is not required in the proposed scenario and therefore its physical interpretation must be taken with caution. We also investigated the timing properties of the source in the framework of existing torque theory, concluding that the observed phenomenology can be consistently explained if the magnetic field of the neutron star is ~10^14 G.Comment: Published as a letter in A&A; 4 pages, 2 figure

The mass of the neutron star in SMC X-1

We present new optical spectroscopy of the eclipsing binary pulsar Sk 160/SMC X-1. From the He I absorption lines, taking heating corrections into account, we determine the radial velocity semi-amplitude of Sk 160 to be 21.8 +/- 1.8 km/s. Assuming Sk 160 fills its Roche-lobe, the inclination angle of the system is i=65.3 deg +/- 1.3 deg and in this case we obtain upper limits for the mass of the neutron star as Mx = 1.21 +/- 0.10 Msolar and for Sk 160 as Mo= 16.6 +/- 0.4 Msolar. However if we assume that the inclination angle is i=90 deg, then the ratio of the radius of Sk 160 to the radius of its Roche-lobe is beta = 0.79 +/- 0.02, and the lower limits for the masses of the two stars are Mx = 0.91 +/- 0.08 Msolar and Mo = 12.5 +/- 0.1 Msolar. We also show that the HeII 4686A emission line tracks the motion of the neutron star, but with a radial velocity amplitude somewhat less than that of the neutron star itself. We suggest that this emission may arise from a hotspot where material accreting via Roche lobe overflow impacts the outer edge of an accretion disc.Comment: Accepted for publication by A&A. 4 Figures & Table 2 will only appear in the on-line versio

Models of X-ray Photoionization in LMC X-4: Slices of a Stellar Wind

We show that the orbital variation in the UV P Cygni lines of the X-ray binary LMC X-4 results when X-rays photoionize nearly the entire region outside of the X-ray shadow of the normal star. We fit models to HST GHRS observations of N V and C IV P Cygni line profiles. Analytic methods assuming a spherically symmetric wind show that the wind velocity law is well-fit by v~(1-1/r)^beta, where beta is likely 1.4-1.6 and definitely <2.5. Escape probability models can fit the observed P Cygni profiles, and provide measurements of the stellar wind parameters. The fits determine Lx/Mdot=2.6+/-0.1 x10^43 erg/s/Msun yr, where Lx is the X-ray luminosity and Mdot is the mass-loss rate of the star. Allowing an inhomogeneous wind improves the fits. IUE spectra show greater P Cygni absorption during the second half of the orbit than during the first. We discuss possible causes of this effect.Comment: 56 pages, 12 figures, to be published in the Astrophysical Journa

The Low X-Ray State of LS 5039 / RX J1826.2-1450

Recent XMM-Newton and Chandra observations of the high mass X-ray binary LS 5039 / RX J1826.2-1450 caught the source in a faint X-ray state. In contrast with previous RXTE observations, we fail to detect any evidence of iron line emission. We also fail to detect X-ray pulsations. The X-ray spectrum can be well fitted by a simple powerlaw, slightly harder than in previous observations, and does not require the presence of any additional disk or blackbody component. XMM-Newton data imply an X-ray photoelectric absorption ($N_{\rm H} \sim 7 \times 10^{21}$ cm$^{-2}$) consistent with optical reddening, indicating that no strong local absorption occurs at the time of these observations. We discuss possible source emission mechanisms and hypotheses on the nature of the compact object, giving particular emphasis to the young pulsar scenario.Comment: 9 pages, 4 figures. Accepted for publication on Astronomy and Astrophysic