Rapidly Alternating Flux States of GX 339-4 During its 2021 Outburst

The low mass X-ray binary GX 339-4 went into an outburst in 2021. At the end of the hard-to-soft transition of this outburst, Insight-HXMT and NuSATR found that the source rapidly alternated between low-flux and high-flux states on a timescale of hours. Swift observations showed that the transition only took around 400s. The high flux states lasted only for a period comparable to the orbital period of the observatory. Time-resolved spectral analysis shows that the sudden changes of flux are confined in the hard X-ray band (>4 keV). The variable non-thermal emission, including the power-law continuum from the corona and the reflected emission from the inner accretion disk, is responsible for the observed variability.

The observed rapidly alternating flux states suggest that the intrinsic power of the corona must change during the state transition. At the end of this presentation, I will discuss possible mechanisms for the observed sudden changes in the coronal power of GX 339-4 and compare the alternating flux states of GX 339-4 with X-ray ‘flip-flops’ observed in other objects, e.g., in terms of their QPOs and spectral states.