Maïmouna Brigitte, Noel Castro Segura, Federico García, Jiří Svoboda, María Díaz Trigo, Mariano Méndez, Federico Vincentelli, Douglas J. K. Buisson, Diego Altamirano

Timing analysis of accreting systems is key to probe 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, forming an accretion disk, and occasionally exhibiting bright eruptions. The BH LMXB Swift J1727.8-1613 (hereafter J1727), recently underwent one of the brightest outbursts ever recorded in X-rays, in August 2023. This analysis aims to study the timing properties of J1727, in the decaying phase of its outburst, using high-time resolution XMM-Newton data. We analyzed J1727's power spectrum (PS) and cross spectrum (CS), which we modeled with Lorentzians. The PS reveals how the source's power is distributed across frequencies, and the Real and Imaginary parts of the CS compare the displacement of the light curves in different energy bands across the observations. Finally, we simultaneously derived the phase lags and the coherence, using a constant phase lag model. While the first (soft-state) observation does not show any 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 towards the hard state, the Imaginary QPO shifts to lower frequencies, the broadband fractional rms amplitude in the 0.3-2 keV energy band increases, while the rms covariance of the Imaginary QPO decreases. Simultaneously, the phase lags increase and the coherence function drops at the Imaginary QPO frequency. In the elusive soft-to-hard transition of J1727, the first XMM-Newton observations of the source reveal an Imaginary QPO also detected in the PS, exhibiting the properties of a type-C QPO.