Discussing the physical meaning of the absorption feature at 2.1 keV in 4U 1538-52

TL;DR

This paper investigates the 2.1 keV absorption feature in the X-ray spectrum of 4U 1538-52, exploring its possible origins related to neutron star atmospheres or stellar wind ions, using high-resolution spectroscopy data.

Contribution

It provides a detailed analysis of the 2.1 keV absorption feature, proposing plausible physical origins and discussing different scenarios based on XMM-Newton observations.

Findings

The feature may originate from atomic transitions in a neutron star atmosphere.

Alternatively, it could be due to ions in the stellar wind.

The analysis supports multiple physical interpretations of the absorption feature.

Abstract

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.