Evolution of the truncation radius in GX 339−4 from XMM–Newton spectral timing

We investigate the changing geometry of the accretion disc in the X-ray binary GX 339−4 using a combination of spectral and timing models to fit six XMM–Newton observations taken during the end of the 2015 X-ray outburst. We use progressively more complex models of the X-ray reflection spectra, from simple disc lines to full relativistic reflection models. For the timing analysis, the Power Spectral Densities (PSDs) are generated from the light curves in the 0.3–0.7 and 0.7–1.5 keV energy bands. We fit PSD models that assume the standard accretion disc truncates at a specific radius, inside of which are two hot-flow zones, one spectrally soft and one spectrally hard, separated by a transition radius. Finally, we combine and jointly fit the full reflection spectral models and the truncated disc PSD models. Our final model is consistent with the spectroscopic and timing data. It suggests that the truncation radius of the disc increases towards the end of the outburst, which is self-consistent with the obtained reflection fraction that is smaller. During this, the source spectrum becomes harder and the soft excess becomes more prominent. The disc truncation radius that increases as the source flux decreases is intermediate between the results of previous studies. However, our analysis of the model performance leads us to believe that higher quality data and better models are required to fully understand the GX 339-4 system and reduce the effect of systematic uncertainties.