  Recent improvements on the sensitivity and spectral resolution of X-ray
observations have led to a better understanding of the properties of matter in
the vicinity of High Mass X-ray Binaries hosting a supergiant star and a
compact object. However the geometry and physical properties of their
environment at larger scales are currently only predicted by simulations. We
aim at exploring the environment of Vela X-1 at a few stellar radii of the
supergiant using spatially resolved observations in the near-infrared and at
studying its dynamical evolution along the 9-day orbital period of the system.
We observed Vela X-1 in 2010 and 2012 using long baseline interferometry at
VLTI, respectively with the AMBER instrument in the K band and the PIONIER
instrument in the H band. The PIONIER observations span through one orbital
period to monitor possible evolutions in the geometry of the system. We
resolved a structure of $8\pm3~R_\star$ from the AMBER data and
$2.0\,_{-1.2}^{+0.7}~R_\star$ from the PIONIER data. From the closure phase we
found that the environment of Vela X-1 is symmetrical. We observed comparable
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 2012, we found the signal to be
constant with the orbital phase within the error bars. We propose three
scenarios for the discrepancy between the two measurements: either there is a
strong temperature gradient in the supergiant wind leading to a hot component
much more compact than the cool part of the wind observed in the K band, or we
observed a diffuse shell in 2010 possibly triggered by an off-state in the
accretion rate of the pulsar that was dissolved in the interstellar medium in
2012, or the structure observed in the H band was the stellar photosphere
instead of the supergiant wind.
