Discovery of two simultaneous non-harmonically related Quasi-Periodic Oscillations in the 2005 outburst of the black-hole binary GRO J1655-40

We studied the low-frequency quasi-periodic oscillations (LFQPOs) in the black hole GRO J1655-40 during the 2005 outburst, using data from the Rossi X-ray Timing Explorer. All LFQPOs could be identified as either type B or type C using previously proposed classification schemes. In the soft state of the outburst the type-C LFQPOs reached frequencies that are among the highest ever seen for LFQPOs in black holes. At the peak of the outburst, in the ultra-luminous state, the power spectrum showed two simultaneous, non-harmonically related peaks which we identified as a type-B and a type-C QPO. The simultaneous presence of a type-C and type-B QPO shows that at least two of the three known LFQPO types are intrinsically different and likely the result of distinct physical mechanisms. We also studied the properties of a broad peaked noise component in the power spectra of the ultra-luminous state. This noise component becomes more coherent with count rate and there are strong suggestions that it evolves into a type-B QPO at the highest observed count rates.

💡 Research Summary

The authors present a comprehensive timing analysis of the 2005 outburst of the black‑hole binary GRO J1655‑40 using data from the Rossi X‑ray Timing Explorer (RXTE). The study focuses on low‑frequency quasi‑periodic oscillations (LFQPOs) in the 0.1–10 Hz range, which are known to be linked to the innermost regions of the accretion flow. By extracting power density spectra (PDS) from 2–26 keV PCA data in 64‑second segments, normalising them according to Leahy et al. (1983), and converting to fractional rms units, the authors model the broad noise components with up to four zero‑centred Lorentzians and fit narrow QPO peaks with additional Lorentzians. They deliberately avoid pre‑subtracting Poisson noise because the high count rates during the ultra‑luminous state (ULS) reduce the Poisson contribution, allowing a cleaner signal‑to‑noise ratio.

Out of 92 selected observations, six distinct PDS types are identified. Types 1–5 correspond to various combinations of flat‑top noise, harmonic QPO families, and broad peaked components. The most striking result is found in PDS type 6 (observation #42), where two narrow peaks appear simultaneously at ≈6 Hz and ≈18 Hz. The lower‑frequency feature exhibits the characteristic properties of a type‑B QPO (moderate quality factor, relatively low rms, and a weak harmonic structure), while the higher‑frequency feature matches the definition of a type‑C QPO (high quality factor, strong rms, and a clear harmonic series). Crucially, the two peaks are not harmonically related, providing the first clear observational evidence that type‑B and type‑C QPOs can coexist in the same time interval.

The outburst evolution is traced through hardness‑intensity diagrams (HID) and rms‑intensity diagrams (RID). The source starts in the low‑hard state (LHS), makes a rapid transition to the soft‑intermediate state (SIMS) within a day, proceeds to the high‑soft state (HSS) for about two months, and then enters the ultra‑luminous state for roughly 20 days. During the ULS the count rate peaks at ~1.1 × 10⁴ cts s⁻¹ PCU⁻¹, and the spectral hardness shows significant variations. In this state a broad peaked noise component appears; its quality factor (Q) increases and its rms amplitude decreases as the count rate rises, suggesting a gradual sharpening of the feature. At the highest count rates this component evolves into a well‑defined type‑B QPO, indicating a possible physical transition from a generic noise process to a coherent oscillation.

The authors also examine the relationship between QPO frequency and power‑law flux. Consistent with earlier work (Motta et al. 2011), type‑C QPO frequencies increase with increasing hard‑flux, whereas type‑B frequencies show little or no correlation with flux. This divergent behaviour supports the hypothesis that the two QPO types arise from distinct mechanisms: type‑C may be linked to Lense‑Thirring precession of a hot inner flow, while type‑B could be associated with a resonance or instability in the accretion disc–corona system.

Overall, the paper delivers three major contributions: (1) the first simultaneous detection of non‑harmonic type‑B and type‑C QPOs, providing decisive evidence that at least two LFQPO classes are intrinsically different; (2) the identification of a broad peaked noise component that sharpens into a type‑B QPO with increasing luminosity, suggesting a continuity between noise and QPO phenomena; and (3) a refined mapping of QPO properties onto spectral state transitions using HID and RID diagnostics. These findings challenge models that treat all LFQPOs as manifestations of a single physical process and motivate the development of more complex theoretical frameworks, possibly involving multiple precessing regions or coupled disc‑corona oscillations. Future work with higher‑resolution timing instruments and advanced numerical simulations will be essential to pinpoint the exact origin of each QPO type and to understand how they interact during rapid state changes in black‑hole binaries.