Detection of a type-C quasi-periodic oscillation during the soft-to-hard transition in Swift J1727.8 --1613

Timing analysis of accreting systems is key to probing the structure and dynamics around compact objects. In black-hole low-mass X-ray binaries (BH LMXBs), the compact object accretes matter from a low-mass companion star via Roche-Lobe overflow and forms an accretion disk that occasionally exhibits bright eruptions. The BH LMXB (hereafter underwent one of the brightest outbursts ever recorded in X-rays in August 2023. We study the timing properties of in the decaying phase of its outburst based on XMM--Newton data with a high-time resolution. We analyzed the power spectrum (PS) and cross spectrum (CS) of which we modeled with Lorentzians. The PS reveals the power distribution of the source across frequencies, and the real and imaginary parts of the CS compare the displacement of the light curves in different energy bands for the different observations. Finally, we simultaneously derived the phase lags and the coherence using a constant phase-lag model. While the first (soft-state) observation shows no strong variability, the two harder observations exhibit quasi-periodic oscillations (QPOs). Because the QPO is more significantly detected in the imaginary part of the CS than in the PS, we refer to it as the "imaginary QPO". The QPO is more prominent in the soft 0.3--2 keV band than in the hard 2--12 keV band. As the source evolves toward the hard state, the imaginary QPO shifts to lower frequencies, the broadband fractional rms amplitude in the 0.3--2 keV energy band increases, and the rms covariance of the imaginary QPO decreases. Simultaneously, the phase lags increase, and the coherence function drops at the imaginary QPO frequency. This analysis provides the first type-C QPO detection in a BH XB during the soft--to--hard transition using XMM--Newton data. The QPO is detected at particularly low energy (0.3--2 keV). Notably, the QPO is significantly detected in the imaginary part of the CS and the PS. Thus, we confirm the physical origin of the coherence drop and the phase-lag excess, which were only observed with NICER before.