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30th of June 2020
X-ray "Flip-flops" in the black hole transient Swift J1658.2-4242
by Bogensberger et al.
"Flip-flops" are top-hat--like X-ray flux variations which have been observed in some transient accreting black hole binary systems, but which remain a poorly understood phenomenon. Bogensberger et al. have detected 15 flip-flops among the multitude of observations of Swift J1658.2-4242 during its outburst in early 2018, enabling a detailed analysis of their individual properties. The team complemented X-ray observation by XMM-Newton, NuSTAR, Astrosat, Swift, Insight-HXMT, and INTEGRAL with ATCA radio observations. As shown in the figure above, on MJD (Modified Julian Day) 58174 (26 Feb 2018), the team observed a major radio flare with ATCA, which reached flux densities ~9 times brighter than those measured at any other time during the outburst. Just 200 seconds after the end of this ATCA observation, the highest NuSTAR and XMM-Newton X-ray fluxes of the entire outburst were measured, 5.4 times brighter than in the previous NuSTAR observation.

The simultaneity of the radio flare with the start of the flip-flop interval implies a possible causal connection between the two effects. Jet ejections, and subsequent radio flares have previously been linked with state transitions, and are characteristic features of X-ray binaries in outburst. However, no other radio flare was seen at any later time, despite observing numerous flip-flops. No radio flare was seen on MJD 58175, when an ATCA observation coincided with a NuSTAR and XMM-Newton measurement of a bright flip-flop state. If all flip-flop transitions are related to jet ejections, a noticeably higher radio flux should have been detected in this observation. The paper will be published in Astronomy & Astrophysics.

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