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

TL;DR

This study reports the detection of a soft lag at the QPO frequency in Swift J1727.8-1613, along with spectral analysis indicating a high inclination angle, providing insights into the accretion flow and QPO origin.

Contribution

First detection of a soft time-lag at the QPO frequency in this source, combined with spectral modeling to estimate the disk inclination and understand accretion flow dynamics.

Findings

Detected a 0.014 s soft lag at QPO frequency.

Estimated disk inclination angle of approximately 85 degrees.

Observed energy-dependent behavior of QPO harmonic strength.

Abstract

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 on 29 August 2023. A promonient type-C Quasi-Periodic Oscillation (QPO) at $0.89 \pm 0.01$ Hz with its harmonic was observed in NICER data of $0.5-10$ keV. Interestingly, the harmonic becomes weaker in the lower energy bands ($0.5-1$ & $1-3$ keV). We also report the first detection of a soft time-lag of $0.014 \pm 0.001$ s at the QPO frequency between harder ($3-10$ kev) and softer ($0.5-3$ keV) band photons observed with the NICER/XTI 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 source to be $\sim 85^\circ$. We discuss how the accretion flow configuration inferred from spectral analysis can help us to understand the origin of QPOs and soft lag in this source.