Broad-band X-ray spectral evolution of GX 339-4 during a state transition

TL;DR

This study analyzes the spectral and timing evolution of GX 339-4 during a 2007 outburst, revealing a state transition characterized by changes in QPO types and high-energy flux, with delays between spectral and timing variations.

Contribution

It provides detailed observational evidence of spectral and timing changes during a state transition in GX 339-4, including the first detection of a delay between spectral parameters and flux variations.

Findings

Transition from hard-intermediate to soft-intermediate state confirmed.

Weak type-A QPO replaces strong type-C QPO during transition.

Significant decrease in high-energy flux observed, with a delay of about one day.

Abstract

We report on X-ray and soft gamma-ray observations of the black-hole candidate GX 339-4 during its 2007 outburst, performed with the RXTE and INTEGRAL satellites. The hardness-intensity diagram of all RXTE/PCA data combined shows a q-shaped track similar to that observed in previous outbursts.The evolution in the diagram suggested that a transition from hard-intermediate state to soft-intermediate state occurred, simultaneously with INTEGRAL observations performed in March. The transition is confirmed by the timing analysis presented in this work, which reveals that a weak type-A quasi-periodic oscillation (QPO) replaces a strong type-C QPO. At the same time, spectral analysis shows that the flux of the high-energy component shows a significant decrease in its flux. However, we observe a delay (roughly one day) between variations of the spectral parameters of the high-energy component and changes in the flux and timing properties. The changes in the high-energy component can be explained either in terms the high-energy cut-off or in terms of a variations in the reflection component. We compare our results with those from a similar transition during the 2004 outburst of GX 339-4.