Rapidly alternating flux states of GX 339-4 during its 2021 outburst captured by Insight-HXMT

Abstract

The low mass X-ray binary GX 339-4 went into a new outburst in 2021. At the end of the hard to soft transition of this outburst, Insight-HXMT found that the source rapidly alternated between low flux and high flux states on a timescale of hours. Two 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 strength of disk thermal emission and the inner radius of the accretion disk are consistent between the two flux states. Assuming the lamppost geometry, our best-fit disk reflection models suggest a very low corona height (within 3 $R_{\mathrm{g}}$) and there is no evidence of significant variation in the corona geometry either. The observed rapidly alternating flux states suggest that the intrinsic power of the corona must change during the state transition. We discuss possible mechanisms for the observed sudden changes in the coronal power of GX 339-4.

10 pages, 7 figures. v2: refereed version

References in corpus (8)

• X-ray Properties of Black-Hole Binaries
• Modelling the behaviour of accretion flows in X-ray binaries
• X-ray reflection in accreting stellar-mass black hole systems
• The mass function of GX 339-4 from spectroscopic observations of its donor star
• High Density Reflection Spectroscopy I. A case study of GX~339-4
• The X-ray binary GX 339-4/V821 Ara: the distance, inclination, evolutionary status and mass transfer
• An underlying clock in the extreme flip-flop state transitions of the black hole transient Swift J1658.2-4242
• MAXI J1820+070 with NuSTAR II. Flaring during the hard to soft state transition with a long soft lag

Cited by in corpus (3)

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• Fast transitions of X-ray variability in the black hole transient GX 339--4: comparison with MAXI J1820+070 and MAXI J1348-630
• About the accuracy of the relxill/relxill_nk models in view of the next generation of X-ray missions