Spectral-Timing Evolution of a Black hole X-ray binary Swift J1727.8-1613: Linking Disk Reflection and Type-C QPO Frequency During the 2023 Outburst

TL;DR

This study analyzes the spectral and timing evolution of the black hole X-ray binary Swift J1727.8-1613 during its 2023 outburst, revealing how accretion geometry and precession influence observed properties.

Contribution

It provides the first detailed spectral-timing analysis linking Lense-Thirring precession to QPO coherence and spectral evolution in this source.

Findings

QPO coherence peaks at intermediate frequencies

Spectral softening correlates with increasing QPO frequency

Disk and corona geometry changes are driven by precession effects

Abstract

We present a comprehensive spectral-timing analysis of a BHXB Swift J1727.8$-$1613 during its 2023 outburst, using five pointed \textit{NuSTAR} observations sampling the luminous hard-intermediate state. Broadband 3-79 keV spectroscopy employs a physically motivated model combining a cool truncated disk (\texttt{diskbb}), relativistic reflection (\texttt{relxill} in reflection-only mode), and Comptonized continuum (\texttt{nthComp}) to probe the inner accretion geometry around a rapidly spinning black hole ($a_\ast\!=\!0.98$) at moderate inclination. Simultaneous timing analysis reveals type-C quasi-periodic oscillations (QPOs) with novel coherence evolution: the quality factor ($Q$) exhibits a striking non-monotonic dependence on both QPO frequency and luminosity, peaking near $\nu_{\rm QPO}\!\sim\!1.2$~Hz and declining at both lower and higher frequencies. This turnover directly constrains Lense-Thirring precession geometry, implying optimal coherence at intermediate truncation radius. A tight photon-index-QPO-frequency correlation demonstrates that spectral softening and frequency rise are concurrent signatures of inward truncation-radius motion. The triadic luminosity evolution-rising disk and Compton, declining reflection-traces precession-driven geometry changes and corona beaming effects. Interpreting disk-normalization variability as apparent-area changes rather than physical radius swings provides new insight into disk-corona boundary layers. These quantitative results provide strong evidence for global Lense-Thirring precession regulation of both timing and spectral properties, establishing Swift J1727.8$-$1613 as a benchmark source for understanding accretion-geometry physics during black hole state transitions.