Evolution of the Inner Accretion Flow in Swift J1727.8-1613 across Intermediate States: Insights from Broadband Spectral and Timing Analysis

Swadesh Chand^1,3*, Andrzej A. Zdziarski², Gulab C. Dewangan³, Pragati Sahu⁴

¹Institute of Astronomy, National Tsing Hua University, Hsinchu, Taiwan
²NCAC, Nicolaus Copernicus Astronomical Center, Warszawa, Poland
³IUCAA, Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, India
⁴Department of Pure and Applied Physic, Guru Ghasidas Vishwavidyalaya (A Central University), Bilaspur, India

* Presenter:Swadesh Chand, email:swadesh.chand@mx.nthu.edu.tw

We present a comprehensive broadband spectral and variability study of the newly detected black
hole X-ray binary Swift J1727.8–1613 in the intermediate states during its 2023 outburst, using multi-
mission observations from NICER, NuSTAR, AstroSat, and Insight-HXMT. The spectral data up to
78 keV in the hard-intermediate state (HIMS) requires models with two Comptonizing regions. In
contrast, models with a single Comptonizing region adequately describe the soft-intermediate states
(SIMS), implying a significant evolution in the disk-corona geometry between the states. The hard
X-ray tail above 100 keV in the HIMS indicates that the electron population in the corona is not
purely thermal but rather hybrid, with a power-law distribution above the thermal cutoff. While both
the reflection modeling and disk continuum fitting favor a truncated disk geometry in the HIMS, the
disk substantially moves close to the innermost stable circular orbit in the SIMS, accompanied by
a significant rise in the disk temperature. This interpretation is further supported by the increase
in the QPO frequency from ~ 1.3 to ~ 6.6 Hz. From joint modeling of the disk continuum and
reflection component, we estimate a black hole mass of ~ 10 M_⊙, spin of ~ 0.8, and disk inclination
angle of 42^ο–50^ο, which match well with the previously reported spectro-polarimetric measurements.
The inferred source distance of ~ 3.5 kpc is consistent with the recent estimate based on optical
spectroscopy. We find a weakly variable or stable disk and a highly variable Comptonized component.

Keywords: Astrophysics, High-Energy Astrophysics, Black Hole X-ray Binary