Search for the Highest-energy Quasiperiodic Oscillation in the Black Hole X-Ray Binary Candidate Swift J1727.8−1613

Abstract We report the first detection of a low-frequency quasiperiodic oscillation (QPO) extending above 250 keV in the black hole X-ray binary candidate Swift J1727.8−1613 using Insight-HXMT observations during its 2023 outburst. Swift J1727.8−1613 is one of the brightest X-ray transients discovered and presents a valuable opportunity for studying high-energy properties of QPOs. Owing to the large effective area of Insight-HXMT in hard X-ray, our observations indicate a remarkably strong QPO signal in the power spectrum above 100 keV. We utilize advanced Hilbert–Huang transform techniques to analyze phase-folded light curves across a wide range of energy bands, observing significant QPOs from 100 to 300 keV in the NaI and CsI detectors, respectively. The detection of QPO profiles above 250 keV can achieve significance levels of ∼8.9 σ for the NaI detector and ∼5.7 σ for the CsI detector. Our results indicate a decrease in QPO fractional rms above 100 keV and an increased soft phase lag with energy, suggesting a geometric origin for the QPOs, likely linked to the precession of a small-scale jet.