Spectral properties of transitions between soft and hard state in GX 339-4

TL;DR

This study analyzes the spectral changes during state transitions of GX 339-4, revealing that the soft intermediate state exhibits significantly harder spectra during the soft-to-hard transition compared to the hard-to-soft transition, with implications for understanding QPOs.

Contribution

The paper provides detailed spectral analysis of GX 339-4 during state transitions, highlighting differences in spectral hardness and QPO properties between transitions, which were not previously characterized in this detail.

Findings

Spectra are harder during the SIMS of the soft-to-hard transition.

Photon index and QPO peak frequencies are lower during the soft-to-hard transition.

Type-B QPOs are associated with different spectral shapes at the same hardness.

Abstract

We present a study of the spectral properties during state transition of GX 339-4. Data are taken from the 2010 outburst of GX 339-4, which is densely covered by Rossi X-ray Timing Explorer, providing an excellent coverage of the state transitions between the low/hard state and the high/soft state. We select all observations within a certain hardness ratio range during the soft intermediate state (SIMS). This sample was chosen in such a way to comprise all observations that show a type-B quasi-periodic oscillation (QPO). In addition, we also investigate the spectra of hard intermediate state observations. The spectra, obtained from Proportional Counter Array data in the 10 to 40 keV range, are fitted with a power law and an additional high energy cut-off if needed. We find that the spectra are significantly harder during the SIMS of the soft-to-hard transition than they are during the hard-to-soft transition. This demonstrates that during the SIMS of the soft-to-hard transition not only the luminosity and peak frequencies of type-B QPOs are lower, but that also the photon index is lower, compared to the hard-to-soft transition. Hence, type-B QPOs can be associated to a different spectral shape even though they appear at the same hardness. However, in each branch only certain combinations of centroid frequency and photon index are realised.