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

¹ Department of Physics, Systems Engineering and Sign Theory, University of Alicante, 03080 Alicante, Spain
e-mail: rodes@dfists.ua.es
² University Institute of Physics Applied to Sciences and Technologies, University of Alicante, 03080 Alicante, Spain
³ Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK

Received: 26 February 2010
Accepted: 7 November 2010

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 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 0.3−11.5 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 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, 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 6.4 keV, which is resolved into two components: a narrow (σ ≤ 10 eV) fluorescence Fe Kα 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.