AstroSat View of Spectral Transition in the Changing-look Active Galaxy NGC 1566 during the Declining Phase of the 2018 Outburst

NGC 1566 is a changing-look active galaxy that exhibited an outburst during 2017-2018 with a peak in 2018 June. We triggered AstroSat observations of NGC 1566 twice in 2018 August and October during its declining phase. Using the AstroSat observations, along with two XMM-Newton observations in 2015 (pre-outburst) and 2018 June (peak outburst), we found that the X-ray power law, the soft X-ray excess, and the disk components showed extreme variability during the outburst. Especially, the soft excess was negligible in 2015 before the outburst, increased to a maximum level by a factor of >200 in 2018 June, and reduced dramatically by a factor of ~7.4 in 2018 August and became undetectable in 2018 October. The Eddington fraction (L/L _Edd) increased from ~0.1% (2015) to ~5% (2018 June) and then decreased to ~1.5% (2018 August) and 0.3% (2018 October). Thus, NGC 1566 made a spectral transition from a high soft-excess state to a negligible soft-excess state at a few percent of the Eddington rate. The soft excess is consistent with warm Comptonization in the inner disk that appears to have developed during the outburst and disappeared toward the end of the outburst over a timescale comparable to the sound-crossing time. The multiwavelength spectral evolution of NGC 1566 during the outburst is most likely caused by the radiation pressure instability in the inner regions of the accretion disk in NGC 1566.