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Description
We report the results of X-ray spectral analysis of the Galactic black hole X-ray binary Swift J1727.8-1613 during its outburst. It showed one of the largest outbursts ever observed in black hole X-ray binaries and was monitored by many X-ray observatories. Using the NICER spectra obtained at various epochs in the outburst, we investigated the accretion disk structure and its evolution. We applied a model consisting of a multi-temperature blackbody component from the accretion disk and a power-law component (which represents Comptonization of the disk photons) to the spectra and found that the model reproduced the data well across different X-ray luminosities. The results revealed that the system made a transition from the low/hard state in the early stages of the outburst to the very high state near the peak luminosity, and then to the high/soft state. Equating the inner disk radius obtained in the high/soft state with the innermost stable circular orbit of the black hole, we estimated the black hole mass to be ~3.4 (D/2.7 kpc) (cos i/cos 0 deg)^(-1/2) M_sun for a non-spinning black hole, and ~20 (D/2.7 kpc) (cos i/cos 0 deg)^(-1/2) M_sun for a maximally spinning black hole.
