Detection of Soft Lag in Swift J1727.8-1613

Dipak Debnath^1,2*, Sujoy Kumar Nath³, Debjit Chatterjee¹, Kaushik Chatterjee⁴, Hsiang-Kuang Chang^1,5

¹Institute of Astronomy, National Tsing Hua University, Hsinchu, Taiwan
²High Energy Astrophysics, Institute of Astronomy Space and Earth Science, Kolkata, India
³High Energy Astrophysics, Indian Centre for Space Physics, Kolkata, India
⁴South-Western Institute For Astronomy Research, Yunnan University, Kunming, China
⁵Department of Physics, National Tsing Hua University, Hsinchu, Taiwan

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

Swift J1727.8-1613 is a Galactic transient black hole candidate (BHC), which was discovered by Swift/BAT on 2023 August 24. It showed the strong signature of quasi-periodic oscillations (QPOs), and their evolutions in hard X-rays. Polarization, relativistic jets, etc. are also detected in this source. We have detected the soft lag of 0.014 ± 0.001 s at the observed QPO frequency (0.89 ± 0.01 Hz) between harder (3-10 keV) and softer (0.5-3 keV) energy bands and estimated inclination angle of the source is at ~ 85 deg. The time lag is one of the important temporal features in BHCs. In hard or positive lags, high energy (hard) photons lag behind low energy (soft) and vice versa is seen in soft or negative lags. Our detection of the soft lag is consistent with earlier detection of soft lags in high inclination (> 60 deg) BHCs, such as XTE J1550-564 and GRS 1915+105. However, there is a debate on the origin of this soft lag, according to Two Component Advective Flow (TCAF) paradigm, reflection of the hard photons (emitted from the 'hot' corona or CENBOL) from the Keplerian disk as soft photons due to down-scattering. These soft photons take a longer route compared to the hard photons directly reached to the observer from the CENBOL. Furthermore, the accretion disk configuration of the high inclination sources is suitable for the detection of the reflected soft photons converting from the hard photons.

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