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

We present a timing analysis of \textit{Insight}-HXMT observations of the black-hole X-ray binary Swift J1727.8$-$1613 across a bright soft X-ray flare on 2023 September 19 (MJD 60206). At the peak of the flare, the source undergoes a brief transition from the hard-intermediate state (HIMS) into the soft-intermediate state (SIMS), marked by the simultaneous appearance of three discrete radio jet ejections, a drop in broadband noise in the 2$-$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 $Q \geq 6$, 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.

💡 Research Summary

The authors present a comprehensive timing study of the black‑hole X‑ray binary Swift J1727.8‑1613 using Insight‑HXMT data taken around a bright soft‑X‑ray flare on 2023 September 19 (MJD 60206). The flare marks a brief transition from the hard‑intermediate state (HIMS) to the soft‑intermediate state (SIMS). Simultaneous multi‑frequency radio observations (RATAN) recorded three discrete jet ejections coincident with the flare, while the X‑ray hardness ratio dropped sharply and broadband noise was strongly suppressed, classic signatures of a SIMS transition.

The timing analysis was performed on both the low‑energy (LE, 2–10 keV) and high‑energy (HE, 28–200 keV) detectors. Light curves were divided into 32‑s segments, Fourier‑transformed, and the resulting power‑density spectra (PDS) and cross‑spectra (CS) were jointly fitted with a set of Lorentzian components using XSPEC and a Markov‑Chain Monte‑Carlo (MCMC) approach (Méndez et al. 2024). A component was deemed significant only if it exceeded a 3σ detection in at least one of the LE PDS, HE PDS, or CS.

Prior to the flare, the source displayed a classic Type‑C QPO: centroid frequencies between 2–10 Hz, quality factors Q ≥ 8, rms amplitudes up to ~15 % and a strong underlying broadband noise. The Type‑C QPO showed the well‑known evolution of phase‑lag spectra, transitioning from hard to soft lags as the frequency increased. At the flare peak, the Type‑C QPO vanished, and a narrow QPO with Q ≥ 6, rms ≤ 5 %, and a characteristic “U‑shaped” phase‑lag spectrum emerged. These properties unambiguously identify it as a Type‑B QPO, traditionally associated only with the SIMS.

The most striking result is that the Type‑B QPO is not confined to the SIMS. During the HIMS intervals before the flare, the authors detect a broad shoulder on the high‑frequency side of the Type‑C peak that matches the frequency range (≈4–6 Hz) and phase‑lag shape of the Type‑B QPO. By jointly fitting the LE and HE PDS and CS, they demonstrate that this shoulder is a distinct Lorentzian component with its own centroid frequency and quality factor, i.e., a Type‑B QPO co‑existing with the Type‑C QPO. After the flare, the Type‑C QPO re‑appears, its frequency slowly rises while its rms declines, and its phase‑lag behavior returns to the hard‑to‑soft pattern typical of Type‑C.

Energy‑dependent analysis across twelve sub‑bands (from 1 keV up to 100 keV) confirms that the two QPOs evolve differently: the Type‑C rms increases with energy and flattens above ~10 keV, whereas the Type‑B rms remains low and shows a weaker energy dependence. The phase‑lag spectra are also distinct: the Type‑C QPO displays a monotonic hard‑to‑soft transition, while the Type‑B QPO exhibits hard lags at low and high energies with a soft‑lag dip at intermediate energies, forming the classic “U”.

The radio data reveal three discrete ejections occurring within the flare window (MJD 60206.19–60206.47). The timing of these ejections aligns with the appearance of the Type‑B QPO and the suppression of broadband noise, reinforcing the long‑standing empirical link between Type‑B QPOs, SIMS transitions, and jet launches. However, the detection of a Type‑B QPO already present in the HIMS challenges the conventional definition of the SIMS as “the state in which a Type‑B QPO appears”.

The authors discuss theoretical implications. Type‑C QPOs are often interpreted as Lense‑Thirring precession of a truncated inner flow or as disc‑corona resonances, both of which naturally produce strong broadband noise and a hard‑to‑soft lag evolution. Type‑B QPOs, by contrast, may arise from a precessing jet base, a transition layer between a jet‑emitting disc and a standard thin disc, or a distinct inner‑flow geometry that becomes dominant when the jet is launched. The simultaneous presence of both QPOs suggests that the inner accretion flow can support two quasi‑stable oscillation modes at once, perhaps reflecting a coupling between the precessing corona (Type‑C) and a nascent jet structure (Type‑B).

Methodologically, the study showcases the power of joint PDS‑CS fitting to uncover weak variability components that would be invisible in standard power‑spectral analysis. By exploiting the full complex cross‑spectrum, the authors achieve precise measurements of phase lags and can separate overlapping Lorentzian features.

In summary, this work provides the first robust detection of co‑existing Type‑B and Type‑C QPOs across the HIMS and SIMS in a canonical black‑hole X‑ray binary. It demonstrates that the physical conditions required for Type‑B QPOs are not exclusive to the SIMS, but can arise earlier in the outburst evolution, likely linked to the inner flow geometry that also governs jet ejection. These findings call for a revision of QPO‑state classification schemes and motivate theoretical models that can accommodate multiple, simultaneous oscillation modes within the same accretion flow.