4.7 Article

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

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OXFORD UNIV PRESS
DOI: 10.1093/mnras/stad969

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binaries: close; X-rays: binaries; X-rays: individual: GX339-4

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We investigated the evolution of quasi-periodic oscillations (QPOs) and noise components using power-density spectra in five snapshots of NICER observations of the black hole transient GX 339-4 during its transition from the hard state to the soft state. We found a strong correlation between the hardness ratio and the type of QPO observed, suggesting a connection between the accretion geometry and spectral properties. We also observed that the type-B QPO undergoes a decoherence process during the transition between different states.
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 by 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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