Stable X-ray reverberation lags in the black hole X-ray binary Swift J1727.8-1613
Aims. We investigate the evolution of X-ray reverberation lags in the black hole X-ray binary Swift J1727.8-1613 during its 2023 outburst, with the aim of probing the inner accretion flow geometry across spectral states. Methods. We analyzed NICER observations covering the low-hard state (LHS) and hard-intermediate state (HIMS). The time lags were computed using Fourier-based techniques, and we constructed lag-frequency and lag-energy spectra. To obtain a robust estimate of the soft lag amplitude, we focused on a frequency range in which the reverberation signal dominates and remains stable, thereby minimizing contamination from hard lags and phase-wrapping effects. Results. The soft lag amplitude increases rapidly from the LHS to the early HIMS and then stays near 10 ms throughout the HIMS. In the frequency range in which reverberation lags prevail, the lag shows little dependence on Fourier frequency. On the other hand, the observed low-frequency lags change from hard-lag dominated to soft-lag dominated, with amplitudes comparable to those measured at higher frequencies. Conclusions. These results suggest that the reverberation lag varies little during the HIMS, consistent with a relatively stable inner accretion geometry during this state. The apparent evolution of the lag amplitude from the LHS to the HIMS can be largely explained by the diminishing effect of hard lags and does not necessarily require significant changes in the intrinsic light-travel timescale. Swift J1727.8-1613 therefore provides a case in which the reverberation signal can be studied with reduced contamination over a broad frequency range, offering new insight into the evolution of the accretion geometry in black hole X-ray binaries.