Detection of QPO Soft Lag during the Outburst of Swift J1727.8-1613: Estimation of Intrinsic Parameters from Spectral Study

The recently discovered bright transient black hole candidate Swift J1727.8-1613 is studied in a broad energy range (0.5–79 keV) using combined NICER and NuSTAR data taken on 2023 August 29. A prominent type C quasiperiodic oscillation (QPO) at 0.89 ± 0.01 Hz with its harmonic was observed in NICER data of 0.5–10 keV. Interestingly, the harmonic becomes weaker in the lower energy bands (0.5–1 and 1–3 keV). We also report the first detection of a soft time lag of 0.014 ± 0.001 s at the QPO frequency between harder (3–10 keV) and softer (0.5–3 keV) band photons observed with the NICER/X-ray timing instrument. This indicates that the inclination of the accretion disk in the binary system might be high. From the detailed spectral analysis with the relxill reflection model, we found the disk inclination angle of the source to be ∼85°. We discuss how the accretion flow configuration inferred from spectral analysis can help us understand the origin of QPOs and soft lag in this source.