Detection of QPO Soft Lag during Outburst of Black Hole Candidate Swift J1727.8-1613
ATel #16287; Dipak Debnath (IASES, Kolkata, India), Sujoy K. Nath (ICSP, Kolkata, India), Debjit Chatterjee (IIA, Bengaluru, India), Kaushik Chatterjee (Yunnan University, Kunming, China), Kaustubha Sen (NTHU. Hsinchu, Taiwan), Hsiang-Kuang Chang (NTHU. Hsinchu, Taiwan)
on 13 Oct 2023; 15:53 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dipak Debnath (dipakcsp@gmail.com)
Subjects: X-ray, Black Hole, Transient
Newly discovered bright X-ray transient source Swift J1727.8-1613 was discovered as a GRB by Swift/BAT on 24 August 2023 (GCN \#34540). Later it has been reported with MAXI, and NICER observations (ATel #16205, #16206). It was classified as a black hole X-ray binary with subsequent observation in X-rays (ATel #16207), optical (ATel #16208) and radio (ATel #16211). Presence of low frequency QPOs are reported using NICER (ATel #16219) and AstroSAT (ATel #16235) data.
We performed preliminary timing analysis using NICER data on 29 Aug 2023. A prominent type-C QPO of frequency ($\nu$) $0.88^{+0.90}{-0.87}$ Hz ($3\sigma$, Q $\sim 5.3 $) and its harmonic at $1.78^{+1.81}{-1.75}$ Hz ($3\sigma$, Q $\sim 4.4 $) in NICER $0.5-10$ keV band were observed. The harmonic was found to be missing in the soft energy band (0.5-1 keV). A soft time lag of $0.014 \pm 0.001$ sec at QPO frequency is observed between hard ($3-10$ keV) and soft ($0.5-3$ keV) energy bands.
The presence of soft lag indicates the source is a high inclination one. Our preliminary spectral analysis with the reflection model ($relxill$) also supports this. We found the inclination angle of the source to be $\sim 87^\circ$.
