  In this paper we present a comprehensive survey of 41 X-ray binaries (10
HMXBs and 31 LMXBs) with Chandra, with specific emphasis on the Fe K region and
the narrow Fe Kalpha line, at the highest resolution possible. We find that: a)
The Fe Kalpha line is always centered at 1.9387 +- 0.0016 Angstroms, compatible
with Fe I up to Fe X; we detect no shifts to higher ionization states nor any
difference between HMXBs and LMXBs. b) The line is very narrow, with FWHM < 5
mAngstroms, which means that the reprocessing material is not rotating at high
speeds. c) Fe Kalpha fluorescence is present in all the HMXB in the survey
while such emissions are very rare (~ 10% ) among LMXBs. d) The lack of Fe line
emission is always accompanied by the lack of any detectable K edge. e) We
obtain the empirical curve of growth of the equivalent width of the Fe Kalpha
line versus the density column of the reprocessing material, i.e. EW_{Kalpha}
vs N_{H}, and show that it is consistent with a reprocessing region spherically
distributed around the compact object. f) We show that fluorescence in X-ray
binaries follows the X-ray Baldwin effect. We interpret this finding as
evidence of decreasing neutral Fe abundance with increasing X-ray illumination
and use it to explain some spectral states of Cyg X-1 and as a possible cause
of the lack of narrow Fe line emission in LMXBs. g) Finally, we study anomalous
morphologies. We present the first evidence of a Compton shoulder in the HMXB
X1908+075. Also the Fe Kalpha lines of 4U1700-37 and LMC X-4 present asymmetric
wings suggesting the presence of highly structured stellar winds in these
systems.
