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We report results from joint NuSTAR, Swift and XMM-Newton observations of thenewly discovered black hole X-ray binary candidate Swift J1658.2-4242 in theintermediate state. We observe a peculiar event in this source, with its X-rayflux rapidly decreasing by $\sim$45\% in $\sim$40~s, accompanied by only subtlechanges in the shape of the broadband X-ray spectrum. In addition, we find asudden turn-on of a transient QPO with a frequency of $6-7$~Hz around the timeof the flux change, and the total fractional rms amplitude of the powerspectrum increases from $\sim$2\% to $\sim$10\%. X-ray spectral and timinganalyses indicate that the flux decrease is driven by intrinsic changes in theaccretion flow around the black hole, rather than intervening material alongthe line of sight. In addition, we do not significantly detect any relativisticdisk reflection component, indicating it is much weaker than previouslyobserved while the source was in the bright hard state. We propose accretiondisk instabilities triggered at a large disk radius as the origin of the fasttransition in spectral and timing properties, and discuss possible causes ofthe unusual properties observed in Swift J1658.2-4242. The prompt fluxvariation detected along with the emergence of a QPO makes the event aninteresting case for investigating QPO mechanisms in black hole X-ray binaries.

astrophysical journal/the astrophysical journal

Broadband X-ray Spectral and Timing Analyses of the Black Hole Binary  Candidate Swift J1658.2-4242: Rapid Flux Variation and the Turn-on of a  Transient QPO