Flip-flop QPO changes during state transitions: a case study of GX339-4 and theoretical discussion

TL;DR

This study examines the rapid flip-flop luminosity changes and QPO behaviour during the 2021 outburst of GX339-4, revealing correlations with spectral properties and exploring potential models for the underlying mechanisms.

Contribution

It provides a detailed analysis of short-timescale flip-flops and QPO variations during a state transition in GX339-4, and discusses possible physical models for these phenomena.

Findings

QPO frequency correlates with count rate and hardness.

Flip-flops occur on timescales as short as 1200 seconds.

Spectral energy distributions remain similar across states.

Abstract

We analyse the 2021 outburst from the black hole X-ray binary GX339-4 observed by NICER around the hard to soft transition, when the system exhibits flip-flops between two distinct luminosity states: a bright state with a 5-6 Hz quasi-periodic oscillation (QPO) and a dim state showing only strong broadband noise. Despite the marked differences in variability patterns between these states, the spectral energy distributions remain strikingly similar, with only minor changes in the black body component in the soft X-ray range. We find that the QPO frequency correlates with the X-ray count rates and hardness, suggesting a tight coupling between the QPO mechanism and the accretion disc's spectral properties. Additionally, we demonstrate that flip-flops can occur on very short timescales, with almost 50 state changes within ~1200 s, while both states can also remain stable over longer periods (at least 1000 s). We explore various QPO models to explain these observations, including the possibility that the corona's accretion speed is near the sound speed, affecting the presence of QPOs. However, the exact mechanism driving the flip-flops and the QPOs remains unclear. Our findings emphasize the complexity of these phenomena and the necessity for further theoretical and observational studies to unravel the intricacies of QPO and flip-flop behaviours in X-ray binaries.