Timing analysis of the black hole candidate Swift J1727.8–1613: Detection of a dip-like feature in the high-energy cross spectrum

¹ Kapteyn Astronomical Institute, University of Groningen, P.O. BOX 800 9700 AV Groningen, The Netherlands
² Instituto Argentino de Radioastronomía (CCT La Plata, CONICET; CICPBA; UNLP), C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina
³ School of Physics and Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK
⁴ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
⁵ University of Chinese Academy of Sciences, Beijing 100049, PR China
⁶ Center for Astrophysics and Space Science, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE

^⋆ Corresponding authors: peijin@astro.rug.nl; mariano@astro.rug.nl

Received: 3 March 2025
Accepted: 28 April 2025

Abstract

We present a timing analysis of observations with the Hard X-ray Modulation Telescope of the black hole X-ray transient Swift J1727.8–1613 during its 2023 outburst. We detect, for the first time in a black hole X-ray binary, a prominent dip at ∼3 − 15 Hz in the real part of the cross spectrum between high-energy (> 25 keV) and low-energy (< 10 keV) photons in the low hard and hard intermediate states, during which the QPO frequency rapidly increases and then stabilizes at ∼1.0 − 1.5 Hz. Remarkably, the real part of the cross spectrum reaches negative values at the frequencies around the minimum of the dip, which is indicative of a phase lag ranging between π/2 and π in this frequency range. We fit the power spectra and the real and imaginary parts of the cross spectra simultaneously using a multi-Lorentzian model. Among the lag models, the Gaussian phase-lag model provides a slightly better reduced χ² than the constant phase-lag and constant time-lag models, while it also alleviates the degeneracy associated with those models. From the parameters of the Lorentzian that fits the dip, we estimated the size of the accretion flow, which consistently exceeds 10 000 km as the QPO frequency increases from 0.13 Hz to 2.0 Hz. Furthermore, both the energy-dependent phase-lag and fractional-rms spectra of the dip exhibit a change in trend around 15 keV, with the phase lag dropping and the rms reaching a local minimum. These spectra closely resemble the shapes predicted by the time-dependent Comptonization model, vKompth, for a low feedback factor, offering a pathway to explain the radiative properties of the corona. Additionally, the coherence function suggests a diversity of variability components potentially arising from different parts of the corona.