  High resolution spectral studies were undertaken at orbital phases 0, 0.25
and 0.5 on the high-mass X-ray binary (HMXB) Vela X-1 using archival Chandra
data. We present (a) the first detailed analysis of the multiple strong narrow
emission lines present in phase 0.5 (b) an analysis of the absorption of the
continuum in phase 0.5, and (c) the first detection of narrow emission and
absorption lines in phase 0.25. Multiple fluorescent and H-and He-like emission
lines in the band 1.6 - 20 Angstrom in eclipse are partially obscured at phase
0.25 by the X-ray continuum. The phase 0.25 spectrum displays 3 triplets, 2
with a blue-shifted resonance (r) line in absorption and the intercombination
(i) and forbidden (f) lines in emission, and shows in absorption other
blue-shifted lines seen in emission in eclipse. At phase 0.5 the soft X-ray
continuum diminishes revealing an "eclipse-like" spectrum, however line flux
values are around 13-fold those in eclipse. We conclude the narrow emission
lines in Vela X-1 become apparent when the continuum is blocked from line of
sight, either by eclipse or by scattering and/or absorption from a wake or
cloud. The H-and He-like lines arise in warm photoionised regions in the
stellar wind, while the fluorescent lines (including a Ni K alpha line) are
produced in cooler clumps of gas outside these regions. Absorption of the 5-13
Angstrom continuum at phase 0.5 may be caused by an accretion wake comprised of
dense stagnant photoionized plasma inside a Stromgren zone. Multiple
fluorescent emission lines may be a common feature of the supergiant category
of HMXBs.
