Long-term X-ray spectral analysis of Cygnus X-1 using AstroSat

We performed a spectral analysis of long-term observations (2016-2019) of the black hole binary Cygnus X-1 using AstroSat, covering its hard, intermediate, and soft states. During soft states, we found that a typical spectrum fitted with a single Comptonization, a thermal disc, and an iron line yields poor fits and unphysically small disk radii (R_in < 1 R_g). While adding a relativistic disk component (kerrbb) improved the fit but required high color factors (f_col ~ 3.35-5.08), indicating additional Comptonization. We therefore adopted a dual-Comptonization model with hot (optically thin) and warm (optically thick) coronae. This provided consistent good fits across all soft and hard states of Cygnus X-1. The spectral evolution reveals the critical role of dual Comptonization in shaping the broadband emission of Cygnus X-1.