  High resolution X-ray spectroscopy is a powerful tool for studying the nature
of the matter surrounding the neutron star in X-ray binaries and its
interaction between the stellar wind and the compact object. In particular,
absorption features in their spectra could reveal the presence of atmospheres
of the neutron star or their magnetic field strength. Here we present an
investigation of the absorption feature at 2.1 keV in the X-ray spectrum of the
high mass X-ray binary 4U 1538-52 based on our previous analysis of the
XMM-Newton data. We study various possible origins and discuss the different
physical scenarios in order to explain this feature. A likely interpretation is
that the feature is associated with atomic transitions in an O/Ne neutron star
atmosphere or of hydrogen and helium like Fe or Si ions formed in the stellar
wind of the donor.
