# High Density Reflection Spectroscopy I. A case study of GX~339-4

## TL;DR

This study uses high density reflection spectroscopy to analyze GX~339-4 across different flux states, revealing significant changes in disc density and consistent near-solar iron abundance.

## Contribution

It demonstrates the application of a high density reflection model to constrain disc density variations in GX~339-4 during different flux states.

## Key findings

- Disc density varies significantly between low and high flux states.
- High density reflection model explains spectra without additional thermal components.
- Iron abundance remains close to solar in different states.

## Abstract

We present a broad band spectral analysis of the black hole binary GX~339-4 with NuSTAR and Swift using high density reflection model. The observations were taken when the source was in low flux hard states (LF) during the outbursts in 2013 and 2015, and in a very high flux soft state (HF) in 2015. The high density reflection model can explain its LF spectra with no requirement for an additional low temperature thermal component. This model enables us to constrain the density in the disc surface of GX~339-4 in different flux states. The disc density in the LF state is $\log(n_{\rm e}/$ cm$^{-3})\approx21$, 100 times higher than the density in the HF state ($\log(n_{\rm e}/$ cm$^{-3})=18.93^{+0.12}_{-0.16}$). A close-to-solar iron abundance is obtained by modelling the LF and HF broad band spectra with variable density reflection model ($Z_{\rm Fe}=1.50^{+0.12}_{-0.04}Z_{\odot}$ and $Z_{\rm Fe}=1.05^{+0.17}_{-0.15}Z_{\odot}$ respectively).