A NuSTAR observation of the low mass X-ray binary GX 349+2 throughout the Z-track

TL;DR

This study uses NuSTAR observations to analyze the spectral states of the Z source GX 349+2, revealing consistent inner disk radii and persistent Fe Kα emission across different states, enhancing understanding of neutron star low mass X-ray binary variability.

Contribution

First detailed spectral analysis of GX 349+2 covering all Z-track states with reflection modeling to determine inner disk radius stability.

Findings

Inner disk radius remains stable at ~17.5 Rg in most states.

Persistent broad Fe Kα line observed across all spectral states.

Inner disk radius during flaring is poorly constrained.

Abstract

Although the most luminous class of neutron star low mass X-ray binaries, known as Z sources, have been well studied, their behavior is not fully understood. In particular, what causes these sources to trace out the characteristic Z-shaped pattern on color-color or hardness-intensity diagrams is not well known. By studying the physical properties of the different spectral states of these sources, we may better understand such variability. With that goal in mind, we present a recent NuSTAR observation of the Z source GX 349+2, which spans approximately 2 days, and covers all its spectral states. By creating a hardness-intensity diagram we were able to extract five spectra and trace the change in spectral parameters throughout the Z-track. GX 349+2 shows a strong, broad Fe K$\alpha$ line in all states, regardless of the continuum model used. Through modeling of the reflection spectrum and Fe K$\alpha$ line we find that in most states the inner disk radius is consistent with remaining unchanged at an average radius of 17.5 $R_g$ or 36.4 km for a canonical 1.4 $M_\odot$ neutron star. During the brightest flaring branch, however, the inner disk radius from reflection is not well constrained.