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

TL;DR

This paper investigates the physical origin of a 2.1 keV absorption feature in the X-ray spectrum of 4U 1538-52, considering various scenarios and ruling out calibration or background effects, but cannot definitively identify its formation region.

Contribution

The study analyzes the nature of the absorption feature in 4U 1538-52 using recent calibrations, providing insights into its possible origins and ruling out some causes.

Findings

Calibration effects are ruled out as the cause.

The feature may originate from the neutron star atmosphere or stellar wind.

The spectral resolution limits definitive identification.

Abstract

The improvement of the capabilities of nowadays X-ray observatories, like Chandra or XMM-Newton, offers the possibility to detect both absorption and emission lines and to study the nature of the matter surrounding the neutron star in X-ray binaries and the phenomena that produce these lines. The aim of this work is to discuss the different physical scenarios in order to explain the meaning of the significant absorption feature present in the X-ray spectrum of 4U 1538-52. Using the last available calibrations, we discard the possibility that this feature is due to calibration, gain effects or be produced by the X-ray background or a dust region. Giving the energy resolution of the XMM-Newton telescope we could not establish if the line is formed in the atmosphere of the neutron star or by the dispersion of the stellar wind of the optical counterpart.