* Astronomy

Title: Footprints in the wind of Vela X-1 traced with MAXI
Authors: V. Doroshenko, A. Santangelo, S. Nakahira, T. Mihara, M. Sugizaki, M. Matsuoka, M. Nakajima, K. Makishima

The stellar wind around the compact object in luminous wind-accreting high mass X-ray binaries is expected to be strongly ionised with the X-rays coming from the compact object. The stellar wind of hot stars is mostly driven by light absorption in lines of heavier elements, and X-ray photo-ionisation significantly reduces the radiative force within the so-called Stroemgren region leading to wind stagnation around the compact object. In close binaries like Vela X-1 this effect might alter the wind structure throughout the system. Using the spectral data from Monitor of All-sky X-ray Image (MAXI), we study the observed dependence of the photoelectric absorption as function of orbital phase in Vela X-1, and find that it is inconsistent with expectations for a spherically-symmetric smooth wind. Taking into account previous investigations we develop a simple model for wind structure with a stream-like photoionisation wake region of slower and denser wind trailing the neutron star responsible for the observed absorption curve.

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Title: Tidal effects on the radial velocity curve of HD77581 (Vela X-1)
Authors: Gloria Koenigsberger, Edmundo Moreno, David Harrington

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.

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