A fractional-variability and phase-lags analysis of Quasi Periodic Oscillations in XTE J1550-564.

We present a detailed timing analysis of the fractional rms variability and phase-lags associated with low frequency quasi-periodic oscillations (QPOs) in the black hole low-mass X-ray binary (LMXB) XTE J1550-564, using archival data from the Rossi X-ray Timing Explorer (RXTE). From an initial sample of 406 RXTE/PCA observations, we identify and select 10 featuring strong QPOs in the 0.3–10 Hz range. Power Density Spectra (PDS) analysis is performed using multi-Lorentzian modeling to isolate the fundamental QPO and its harmonic components. We further investigate the energy dependence of both rms amplitude and phase-lags across the QPO frequency ranges, with a focus on Type-B and Type-C QPOs. Phase-lags are extracted and analyzed to distinguish between hard and soft lag behavior, probing Comptonization and geometric effects in the innermost regions of the accretion flow. Additionally, we explore correlations between the fundamental QPO parameters and spectral components by modeling the fractional variability (rms) and phase lags spectra with Vkompth variable thermal Comptonization model. This allows us to constrain the dependence of the corona’s electron temperature, size, feedback fraction, and photon index on the QPO’s centroid frequency. Our results provide observational constraints on the physical properties of the accretion disc corona and support models in which QPOs originate from a coherent modulation of the Comptonizing region. The combined spectral-timing approach adopted here contributes to a more unified picture of variability in black hole X-ray binaries (BHXBs).