  The improvement of the capabilities of nowadays X-ray observatories, like
Chandra or XMM-Newton, offers the possibility to detect both absorption and
emission lines and to study the nature of the matter surrounding the neutron
star in X-ray binaries and the phenomena that produce these lines. The aim of
this work is to discuss the different physical scenarios in order to explain
the meaning of the significant absorption feature present in the X-ray spectrum
of 4U 1538-52. Using the last available calibrations, we discard the
possibility that this feature is due to calibration, gain effects or be
produced by the X-ray background or a dust region. Giving the energy resolution
of the XMM-Newton telescope we could not establish if the line is formed in the
atmosphere of the neutron star or by the dispersion of the stellar wind of the
optical counterpart.
