The evolution of GX 339-4 in the low-hard state as seen by NuSTAR and Swift

TL;DR

This study uses NuSTAR and Swift data to analyze the evolution of the accretion disk in GX 339-4 during low-hard states, revealing disk truncation behavior at low luminosities and constraining coronal properties.

Contribution

It provides the first detailed spectral analysis of GX 339-4's low-hard state across different outburst phases using the relxillCp model, highlighting disk truncation.

Findings

Disk becomes more truncated at lower luminosities.

Maximum truncation radius observed is 37 gravitational radii.

Consistent results between reflection and coronal Comptonization models.

Abstract

We analyze eleven NuSTAR and Swift observations of the black hole X-ray binary GX 339-4 in the hard state, six of which were taken during the end of the 2015 outburst, five during a failed outburst in 2013. These observations cover luminosities from 0.5%-5% of the Eddington luminosity. Implementing the most recent version of the reflection model relxillCp, we perform simultaneous spectral fits on both datasets to track the evolution of the properties in the accretion disk including the inner edge radius, the ionization, and temperature of the thermal emission. We also constrain the photon index and electron temperature of the primary source (the "corona"). We find the disk becomes more truncated when the luminosity decreases, and observe a maximum truncation radius of $37R_g$. We also explore a self-consistent model under the framework of coronal Comptonization, and find consistent results regarding the disk truncation in the 2015 data, providing a more physical preferred fit for the 2013 observations.