A test of the nature of the Fe K Line in the neutron star low-mass X-ray binary Serpens X-1

TL;DR

This study uses high-resolution X-ray spectroscopy to analyze the Fe K emission line in the neutron star low-mass X-ray binary Serpens X-1, providing evidence that relativistic reflection models better explain the line's broadening.

Contribution

It presents the longest high-resolution Chandra spectrum of a neutron star LMXB and demonstrates that relativistic reflection models outperform Gaussian fits for the Fe K line.

Findings

Relativistic reflection model fits the Fe line better than Gaussian.

No strong evidence for additional narrow lines in the spectrum.

High spectral resolution confirms the reflection origin of the Fe K line.

Abstract

Broad Fe K emission lines have been widely observed in the X-ray spectra of black hole systems, and in neutron star systems as well. The intrinsically narrow Fe K fluorescent line is generally believed to be part of the reflection spectrum originating in an illuminated accretion disk, and broadened by strong relativistic effects. However, the nature of the lines in neutron star LMXBs has been under debate. We therefore obtained the longest, high-resolution X-ray spectrum of a neutron star LMXB to date with a 300 ks Chandra HETGS observation of Serpens X-1. The observation was taken under the "continuous clocking" mode and thus free of photon pile-up effects. We carry out a systematic analysis and find that the blurred reflection model fits the Fe line of Serpens X-1 significantly better than a broad Gaussian component does, implying that the relativistic reflection scenario is much preferred. Chandra HETGS also provides highest spectral resolution view of the Fe K region and we find no strong evidence for additional narrow lines.