A systematic analysis of the broad Fe Kalpha line in neutron star LMXBs with XMM-Newton

TL;DR

This study systematically analyzed XMM-Newton data of 16 neutron star low-mass X-ray binaries, revealing consistent Fe Kalpha emission lines with no relativistic asymmetry, and highlighting the importance of proper data correction methods.

Contribution

It provides a comprehensive, corrected analysis of Fe Kalpha lines in NS LMXBs, challenging previous claims of relativistic effects based on line asymmetries.

Findings

80% of observations showed significant Fe line emission

Average line centroid at 6.67 keV with a width of 0.33 keV

Lines fit well with simple Gaussian profiles, no relativistic asymmetry

Abstract

We analysed the XMM-Newton archival observations of 16 neutron star (NS) low-mass X-ray binaries (LMXBs) to study the Fe K emission in these objects. The sample includes all the observations of NS LMXBs performed in EPIC pn Timing mode with XMM-Newton publicly available until September 30, 2009. We performed a detailed data analysis considering pile-up and background effects. The properties of the iron lines differed from previous published analyses due to either incorrect pile-up corrections or different continuum parameterization. 80% of the observations for which a spectrum can be extracted showed significant Fe line emission. We found an average line centroid of 6.67 $\pm$ 0.02 keV and a finite width, $\sigma$, of 0.33 $\pm$ 0.02 keV. The equivalent width of the lines varied between 17 and 189 eV, with an average weighted value of 42 $\pm$ eV. For sources where several observations were available the Fe line parameters changed between observations whenever the continuum changed significantly. The line parameters did not show any correlation with luminosity. Most important, we could fit the Fe line with a simple Gaussian component for all the sources. The lines did not show the asymmetric profiles that were interpreted as an indication of relativistic effects in previous analyses of these LMXBs.