Absence of Reflection Features in NuSTAR Spectra of the Luminous Neutron Star X-Ray Binary GX 5-1

• Published in: The Astrophysical journal Vol. 853; no. 2; pp. 157 - 162
• Main Authors: Homan, Jeroen, Steiner, James F., Lin, Dacheng, Fridriksson, Joel K., A. Remillard, Ronald, Miller, Jon M., Ludlam, Renee M.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.02.2018; IOP Publishing
• Subjects: accretion, accretion disks; Astrophysics; Binary stars; Continuum modeling; Deposition; Luminosity; Neutron stars; Neutrons; Reflection; Spectra; Stars & galaxies; stars: neutron; Stellar systems; X ray binaries; X ray stars; X-rays: binaries; X-rays: individual (GX 5-1)
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aaa439

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.