2026ApJ..1000..137C

We present a comprehensive broadband spectral and variability study of the newly detected black hole X-ray binary Swift J1727.8-1613 in the intermediate states during its 2023 outburst, using multimission observations from NICER, NuSTAR, AstroSat, and Insight-HXMT. Spectral data up to 78 keV in the hard-intermediate state (HIMS) require models with two Comptonizing regions. In contrast, models with a single Comptonizing region adequately describe the soft-intermediate states (SIMS), implying a significant evolution in the disk-corona geometry between the states. The hard X-ray tail above 100 keV in the HIMS, detected with both AstroSat/CZTI and Insight-HXMT/High-Energy X-ray Telescope, indicates that the electron population in the corona is not purely thermal but rather hybrid, with a power-law distribution above the thermal cutoff. While both the reflection modeling and disk-continuum fitting favor a truncated disk geometry in the HIMS, the disk in the SIMS moves substantially closer to the innermost stable circular orbit, accompanied by a significant rise in disk temperature. This interpretation is further supported by the increase in the quasiperiodic oscillation frequency from ∼1.3 to ∼6.6 Hz. From joint modeling of the disk continuum and reflection component and assuming the distance of 3.4 kpc, we estimate a black hole mass of $10.{3}_{-2.5}^{+5.5}\,{M}_{\odot }$, spin of $0.7{9}_{-0.15}^{+0.07}$, and the disk inclination angle of ∼37°-53°, which match well with the previously reported spectropolarimetric measurements. We find a weakly variable or stable disk and a highly variable Comptonized component.