Discussing the physical meaning of the absorption feature at 2.1 keV in   4U 1538-52
  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. 
J.J. Rodes-Roca, J.M. Torrej\'on, S. Mart\'inez-N\'u\~nez, A.   Gim\'enez-Garc\'ia, G. Bernab\'eu
astro-ph.HE