Kavli Affiliate: Luis C. Ho
| First 5 Authors: Jie Liao, Ning Chang, Lang Cui, Pengfei Jiang, Didong Mou
| Summary:
Type-C quasi-periodic oscillations (QPOs) in black hole X-ray transients
typically manifest in the low-hard and hard-intermediate states. This study
presents a detailed spectral and temporal analysis of the black hole candidate
Swift J1727.8-1613 using NICER observations from August and September 2023,
with a focus on the first flare period. The time-averaged spectra, along with
the rms and phase-lag spectra of the QPOs, were jointly fitted using the
time-dependent Comptonization model text{vkompthdk} to examine the geometry of
the corona during this flare. The results provide a comprehensive view of the
QPO, where we detected type-C QPOs with a centroid frequency increasing from
0.32 Hz to 2.63 Hz, while it elevated when it entered the flare state, and its
energy spectral properties as they evolved during the first flare period.
Correlations between spectral and temporal properties suggest that type-C QPOs
are primarily modulated by Lense-Thirring precession. Based on simultaneous
radio observations indicating discrete jet ejections, we propose, for the first
time, a scenario where the temporarily extended corona contracts vertically
from approximately 2714 km to less than 900 km, overlying the inner accretion
disc, with a transient jet seemingly being launched. The corona then recovers
to nearly 2000 km by the end of the first flare period of Swift J1727.8-1613,
rather than undergoing horizontal changes. A phenomenological analysis of the
corona scenario during this flare period was also conducted.
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