2011A&A...526A..64R

The properties of the X-ray emission lines are a fundamental tool for studying the nature of the matter surrounding the neutron star and the phenomena that produce these lines. The aim of this work is to analyse the X-ray spectrum of 4U 1538-52 obtained by the XMM-Newton observatory and to look for the presence of diagnostic lines in the energy range 0.3-11.5keV. We used a 54 ks PN & MOS/XMM-Newton observation of the high-mass X-ray binary 4U 1538-52 covering the orbital phase between 0.75 to 1.00 (the eclipse ingress). We modelled the 0.3-11.5keV continuum emission with three absorbed power laws and looked for the emission lines. We found previously unreported recombination lines in this system at ∼2.4keV, ∼1.9keV, and ∼1.3keV, which is consistent with the presence of highly ionized states of S XV Heα, Si XIII Heα, and either Mg Kα or Mg XI Heα. On the other hand, in spectra that are both out of eclipse and in eclipse, we detect a fluorescence iron emission line at 6.4keV, which is resolved into two components: a narrow (σ≤10eV) fluorescence Fe Kα line plus one hot line from highly photoionized Fe XXV. The detection of new recombination lines during eclipse ingress in 4U 1538-52 indicates that there is an extended ionized region surrounding the neutron star.