Absence of reflection features in NuSTAR spectra of the luminous neutron star X-ray binary GX 5-1

TL;DR

This study analyzes NuSTAR spectra of the luminous neutron star binary GX 5-1, finding no reflection features, likely due to a highly ionized accretion disk at near-Eddington luminosities.

Contribution

First detailed NuSTAR spectral analysis of GX 5-1 showing absence of reflection features, suggesting high ionization as a key factor.

Findings

No reflection features detected in spectra

High ionization state likely explains the absence

GX 5-1's luminosity exceeds Eddington limit

Abstract

We present NuSTAR observations of the luminous neutron star low-mass X-ray binary (NS LMXB) and Z source GX 5-1. During our three observations made with separations of roughly two days, the source traced out an almost complete Z track. We extract spectra from the various branches and fit them with a continuum model that has been successfully applied to other Z sources. Surprisingly, and unlike most of the (luminous) NS-LMXBs observed with NuSTAR, we do not find evidence for reflection features in any of the spectra of GX 5-1. We discuss several possible explanations for the absence of reflection features. Based on a comparison with other accreting neutron star systems and given the high luminosity of GX 5-1 (~1.6-2.3 times the Eddington luminosity, for a distance of 9 kpc), we consider a highly ionized disk the most likely explanation for the absence of reflection features in GX 5-1.