Detecting emission lines with XMM-Newton in 4U 1538-52

TL;DR

This study analyzes XMM-Newton X-ray data of 4U 1538-52, revealing new recombination lines and detailed iron emission features, enhancing understanding of the ionized environment around the neutron star.

Contribution

It reports the first detection of recombination lines at specific energies in 4U 1538-52, indicating an extended ionized region around the neutron star.

Findings

Detection of recombination lines at 2.4 keV, 1.9 keV, and 1.3 keV.

Observation of a resolved iron K? emission line with two components.

Presence of highly ionized states of sulfur, silicon, and magnesium.

Abstract

Context. 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. Aims. The aim of this work is to analyze 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 0.3-11.5 keV. Methods. 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 have modelled the 0.3-11.5 keV continuum emission with three absorbed power laws and looked for the emission lines. Results. We found previously unreported recombination lines, in this system, at 2.4 keV, 1.9 keV and 1.3 keV, consistent with the presence of highly ionized states of S XV He?, Si XIII He? and Mg K? or Mg XI He?. On the other hand, both out of eclipse and in eclipse we detect a fluorescence iron emission line at 6.4 keV which is resolved into two components: a narrow ($\sigma \leq$ 10 eV) fluorescence Fe K? line plus one hot line from highly photoionized Fe XXV. Conclusions. 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.