Black-hole X-ray binary Swift J1727.8–1613 shows simultaneous Type-B and Type-C quasiperiodic oscillations across the hard-intermediate and soft-intermediate states

¹ Kapteyn Astronomical Institute, University of Groningen P.O. Box 800 9700 AV Groningen, The Netherlands
² Instituto Argentino de Radioastronomía (CCT La Plata, CONICET, CICPBA, UNLP), C.C.5 (1894) Villa Elisa Buenos Aires, Argentina
³ School of Physics and Astronomy, University of Southampton Southampton Hampshire SO17 1BJ, UK
⁴ Fluid and Complex Systems Centre, Coventry University Coventry CV1 5FB, UK
⁵ INAF Istituto di Astrofisica e Planetologia Spaziali Via del Fosso del Cavaliere 100 00133 Roma, Italy

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Received: 11 June 2025
Accepted: 19 November 2025

Abstract

We present a timing analysis of Insight-HXMT observations of the black-hole X-ray binary Swift J1727.8−1613 across a bright soft X-ray flare on September 19, 2023 (MJD 60206). At the peak of the flare, the source undergoes a brief transition from the hard-intermediate state (HIMS) into the soft-intermediate state (SIMS), marked by the simultaneous appearance of three discrete radio jet ejections, a drop in broadband noise in the 2−10 keV band, and the presence of a narrow quasi-periodic oscillation (QPO) with a characteristic “U”-shaped phase-lag spectrum and a quality factor of Q ≥ 6, features that robustly identify it as a Type-B QPO. The Type-C QPO, which was clearly detected in the HIMS prior to the flare, is not observed at the flare’s peak and only reappears afterward. Most notably, we find that the Type-B QPO is not restricted to the SIMS: it is present throughout all our observations, including those taken in the HIMS, where it appears as a broad shoulder of the Type-C QPO. During the flare, the Type-B and Type-C QPOs exhibit distinct evolutionary trends in frequency, fractional rms amplitude, and phase lag. These results challenge the traditional view that Type-B QPOs are exclusive to the SIMS, a state that is, in fact, defined by their appearance in the power spectrum, and directly linked to discrete jet ejections. Instead, our findings suggest that the physical conditions giving rise to Type-B QPOs occur more broadly within the inner accretion flow.