Studying the X-ray hysteresis in GX 339-4: the disc and iron line over one decade

TL;DR

This study analyzes 11 years of RXTE data on GX 339-4, revealing hysteresis in X-ray spectral features, disc behavior consistent with theoretical models, and similarities to AGN in a newly constructed Disc Fraction Luminosity Diagram.

Contribution

It provides a comprehensive analysis of X-ray spectral evolution, including iron line behavior and disc temperature decay, and introduces the first Disc Fraction Luminosity Diagram for an X-ray binary.

Findings

Disc temperature decay follows T^4.75, suggesting a nearly constant inner disc radius.

Hysteresis observed in iron line width and equivalent width with flux.

Disc Fraction Luminosity Diagram resembles that of active galactic nuclei.

Abstract

We report on a comprehensive and consistent investigation into the X-ray emission from GX 339-4. All public observations in the 11 year RXTE archive were analysed. Three different types of model - single powerlaw, broken powerlaw and a disc + powerlaw - were fitted to investigate the evolution of the disc, along with a fixed gaussian component at 6.4 keV to investigate any iron line in the spectrum. We show that the relative variation in flux and X-ray colour between the two best sampled outbursts are very similar. The decay of the disc temperature during the outburst is clearly seen in the soft state. The expected decay is S_Disc \propto T^4; we measure T^4.75\pm0.23. This implies that the inner disc radius is approximately constant in the soft state. We also show a significant anti-correlation between the iron line significant width and the X-ray flux in the soft state while in the hard state the EW is independent of the flux. This results in hysteresis in the relation between X-ray flux and both line flux and EW. To compare the X-ray binary outburst to the behaviour seen in AGN, we construct a Disc Fraction Luminosity Diagram for GX 339-4, the first for an X-ray binary. The shape qualitatively matches that produced for AGN. Linking this with the radio emission from GX 339-4 the change in radio spectrum between the disc and power-law dominated states is clearly visible.