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arXiv · 2304.06077

A journey from the hard to the soft state: How do QPOs evolve in the 2021 outburst of GX 339-4?

H. Stiele·A. K. H. Kong
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Abstract

We investigated the snapshots of five NICER observations of the black hole transient GX 339-4 when the source transited from the hard state into the soft state during its outburst in 2021. In this paper, we focused our study on the evolution of quasi-periodic oscillations (QPOs) and noise components using power-density spectra. In addition, we derived hardness ratios comparing count rates above and below 2 keV. The evolution from the hard to the soft state was a somewhat erratic process showing several transitions between states that are dominated by top-flat noise and can show type-C QPOs; those that are dominated by red noise and can show type-B QPOs. From the parameters that we studied, we only found a strong correlation between the hardness ratio and the type of QPO observed. This implies that the appearance of type-B QPOs is related to a change in the accretion geometry of the system that also reflects in altered spectral properties. We also observed that the type-B QPO forms from or disintegrates into a broad peaked feature when the source comes out of or goes to the hard-intermediate state, respectively. This implies some strong decoherence in the process that creates this feature.

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H. Stiele, A. K. H. Kong. 2023-04-12. A journey from the hard to the soft state: How do QPOs evolve in the 2021 outburst of GX 339-4?. https://doi.org/10.1093/mnras%2Fstad969

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