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

TL;DR

This study analyzes Insight-HXMT observations of Swift J1727.8-1613 during a bright soft X-ray flare to test whether Type-B QPOs are exclusive to the SIMS. By jointly fitting power and cross spectra across energy bands with Lorentzian components and using MCMC to quantify uncertainties, the authors disentangle two independent QPOs: the Type-C QPO associated with a disc–corona region and a previously overlooked Type-B QPO linked to a jet-base corona. They find that the Type-B QPO persists across the hard-intermediate state and becomes dominant during the flare, with $\nu_B \sim 1.1\nu_C$ and distinct positive phase lags, while the Type-C QPO fades, challenging the paradigm that Type-B QPOs require SIMS and jet ejections. The results imply a broader, energy- and geometry-dependent origin for Type-B QPOs and highlight the value of joint PDS-CS analysis for revealing weak or overlapping timing components and tracing inner accretion-flow geometry.

Abstract

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.

Figures (14)

• Figure 1: Ratan radio and Insight-HXMT X-ray observations of Swift J1727.8$-$1613 in the period MJD 60195-60215. Each point corresponds to an individual GTI. From top to bottom, the panels show, respectively, the Ratan radio fluxes at three frequencies 2024ApJ...968...76I, the LE $2-10$ keV light curve, the HE $28-200$ keV light curve and the corresponding hardness ratio. The green shaded region indicates the observations we use in this paper.
• Figure 2: Zoom-in of the LE light curve of Swift J1727.8$-$1613 between MJD 60204 and 60209 in Fig. \ref{['fig:flare_light']}. The gray shaded area marks the time, considering the uncertainty, of the jet ejections reported by 2025ApJ...984L..53W, where a simultaneous soft X-ray flare occurs. The dashed vertical lines indicate the seven observations shown in Table \ref{['tab:rep_obs']} and Fig. \ref{['fig:flare_representative_observations']}.
• Figure 3: PDS and the CS of Obs #5 in Table \ref{['tab:rep_obs']}. Left panels: LE $2-10$ keV PDS (black) and HE $28-200$ keV PDS (green). Right panels: The real (black) and imaginary (green) parts of the CS between the HE and LE data.
• Figure 4: PDS and CS of the seven representative Insight-HXMT observations marked with vertical lines in Fig. \ref{['fig:flare_light_zoomin']}, and given in Table \ref{['tab:rep_obs']}. Left panels: LE $2-10$ keV PDS (black) and HE $28-200$ keV PDS (green). Right panels: The real (black) and imaginary (green) parts of the CS. The individual Lorentzians are shown as dashed-dotted lines in the LE PDS and dashed lines in the HE PDS; in the CS, the dashed-dotted lines indicate the real part, and the dashed lines indicate the imaginary part. We highlight the Type-C (red) and Type-B (blue) QPOs in the PDS and the CS. Data with absolute values greater than or equal to $5\times10^{-3}$ are scaled logarithmically, while values with absolute values smaller than $5\times10^{-3}$ are scaled linearly.
• Figure 5: Parameters of the Type-C and Type-B QPOs of the observations of Swift J1727.8$-$1613 in Fig. \ref{['fig:flare_light_zoomin']}. Top panel: the QPO frequency. Middle panel: $1/Q$. Bottom panel: the phase lag. The gray area marks the time of the soft X-ray flare and the jet ejections reported by 2025ApJ...984L..53W.