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^⋆

¹ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris-Diderot, Paris Sciences et Lettres, 5 place Jules Janssen, 92195 Meudon, France
e-mail: choquet@stsci.edu
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD-21218, USA
³ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
⁴ European Southern Observatory, Alonzo de Córdova 3107, 19001 Casilla, Santiago 19, Chile
⁵ Instituto de Astronomia, Geofìsica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, SP 05508-900 São Paulo, Brazil
⁶ Max-Planck-Institut für Astronomie, Königstuhl 1, 69117 Heidelberg, Germany

Received: 28 May 2013
Accepted: 4 November 2013

Abstract

Context. Recent improvements in the sensitivity and spectral resolution of X-ray observations have led to a better understanding of the properties of matter in the near vicinity of high-mass X-ray binaries (HMXB) hosting a supergiant star and a compact object. However, the geometry and physical properties of their environments on larger scales (up to a few stellar radii) are currently only predicted by simulations but have never been directly observed.

Aims. We aim to explore the environment of Vela X-1 at a few stellar radii (R_⋆) of the supergiant using spatially resolved observations in the near-infrared, and to study its dynamical evolution along the nine-day orbital period of the system.

Methods. We observed Vela X-1 in 2010 and 2012 using near-infrared long baseline interferometry at the Very Large Telescope Interferometer (VLTI), respectively with the AMBER instrument in the K band (medium spectral resolution), and the PIONIER instrument in the H band (low spectral resolution). The PIONIER observations span one orbital period to monitor possible evolutions in the geometry of the system.

Results. We resolved a structure of 8 ± 3  R_⋆ from the AMBER K-band observations, and 2.0_-1.2^+0.7R_* from the PIONIER H-band data. From the closure phase observable, we found that the circumstellar environment of Vela X-1 is symmetrical in the near-infrared. We observed comparable interferometric measurements between the continuum and the spectral lines in the K band, meaning that both emissions originate from the same forming region. From the monitoring of the system over one period in the H band in 2012, we found the signal to be constant with the orbital phase within the error bars.

Conclusions. We propose three possible scenarios for this discrepancy between the two measurements: 1) there is a strong temperature gradient in the supergiant wind, leading to a hot component that is much more compact than the cool part of the wind observed in the K band; 2) we observed a diffuse shell in 2010, possibly triggered by an off-state in the accretion rate of the neutron star that was dissolved in the interstellar medium in 2012 during our second observations; or 3) the structure observed in the H band was the stellar photosphere instead of the supergiant wind.