Accretion Flow Properties of Swift J1727.8–1613 during Its Rising Phase of the Discovery Outburst: Estimation of Mass from Spectrotemporal Study

Abstract The rising phase of the 2023–24 outburst of the recently discovered bright transient black hole candidate Swift J1727.8–1613 was monitored by Insight-HXMT. We study the evolution of hard-band (4–150 keV) and soft-band (2–4 keV) photon count rates, the hardness ratio (HR), quasiperiodic oscillation (QPO) frequencies, and spectral features using daily observations from the HXMT/LE, ME, and HE instruments between 2023 August 25 and October 5. The QPO frequency is found to be strongly correlated with the soft-band X-ray count rates and spectral photon indices. In contrast, a strong anticorrelation is observed between HR and QPO frequency, as well as between HR and photon index. Based on the evolution of the temporal and spectral properties, the rising phase of the outburst is subdivided into six parts. The evolution of the QPOs in parts 1–5 is fitted with the propagating oscillatory shock solution to understand the nature of the evolution from a physical perspective. An inward-propagating shock with weakening strength (except in part 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.