  Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique
opportunity to study accretion onto a neutron star from clumpy winds of O/B
stars and to disentangle the complex accretion geometry of these systems. In
Chandra-HETGS spectroscopy at orbital phase ~0.25, when our line of sight
towards the source does not pass through the large-scale accretion structure
such as the accretion wake, we observe changes in overall spectral shape on
timescales of a few kiloseconds. This spectral variability is, at least in
part, caused by changes in overall absorption and we show that such strongly
variable absorption cannot be caused by unperturbed clumpy winds of O/B stars.
We detect line features from high and low ionization species of silicon,
magnesium and neon whose strengths and presence depend on the overall level of
absorption. They imply a co-existence of cool and hot gas phases in the system
that we interpret as a highly variable, structured accretion flow close to the
compact object such as has been recently seen in simulations of wind accretion
in high-mass X-ray binaries.
