Accretion Flow Properties of Swift J1727.8-1613

Dipak Debnath^1,2*, Hsiang-Kuang Chang^1,3, Lev Titarchuk⁴

¹Institute of Astronomy, National Tsing Hua University, Hsinchu, Taiwan
²High Energy Astrophysics, Institute of Astronomy Space and Earth Science, Kolkata, India
³Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁴Dipartimento di Fisica, University di Ferrara, Ferrara, Italy

* Presenter:Dipak Debnath, email:dipakcsp@gmail.com

Swift J1727.8-1613 is a Galactic transient black hole candidate (BHC) that was discovered by Swift/BAT on 2023 August 24. The source exhibited strong signatures of quasi-periodic oscillations (QPOs) and their evolution in hard X-rays. Polarization and relativistic jets have also been detected from this source. We present a detailed study of the spectral and temporal properties of the source during the initial rising phase of its outburst using broadband (2-150 keV) data from Insight-HXMT. The observed low-frequency QPOs are found to be strongly correlated with the soft-band (2-4 keV) X-ray count rates and the spectral photon indices. In contrast, a strong anti-correlation is observed between the hardness ratio (defined as the ratio of 4-150 keV to 2-4 keV count rates) and the QPO frequency, as well as between the hardness ratio and the spectral photon index. Based on the evolution of the temporal and spectral properties, the rising phase of the outburst is subdivided into six distinct phases. The evolution of the QPOs in phases 1-5 is fitted with the propagating oscillatory shock (POS) solution to understand the nature of the evolution from a physical perspective. An inward-propagating shock with weakening strength (except in phase 4) is observed during the period of our study. The probable mass of the source is estimated to be 13.5 ± 1.9 M_⊙ using the QPO frequency (ν)-photon index (Γ) scaling method.

Keywords: X-ray binary stars, X-ray transient sources, Black holes, Black hole physics, Accretion