Thermal Comptonization in a Changing Corona in the Changing-look Active Galaxy NGC 1566

We present broadband UV/X-ray spectral variability of the changing-look active galactic nucleus (AGN) NGC 1566, based on simultaneous near-ultraviolet and X-ray observations performed by the XMM-Newton, Swift, and NuSTAR satellites at five different epochs during the declining phase of the 2018 outburst. We found that the accretion disk, soft X-ray excess, and X-ray power-law components were extremely variable. Additionally, the X-ray power-law flux was correlated with both the soft excess plus disk and the pure disk fluxes. Our finding shows that at high-flux levels the soft X-ray excess and the disk emission both provided the seed photons for thermal Comptonization in the hot corona, whereas at low-flux levels, where the soft excess was absent, the pure disk emission alone provided the seed photons. The X-ray power-law photon index was only weakly variable (Delta Gamma(hot) <= 0.06), and it was not well correlated with the X-ray flux over the declining timescale. On the other hand, we found that the electron temperature of the corona increased from similar to 22 to similar to 200 keV with the decreasing numbers of seed photons from 2018 June to 2019 August. At the same time, the optical depth of the corona decreased from tau (hot) similar to 4 to similar to 0.7, and the scattering fraction increased from similar to 1% to similar to 10%. These changes suggest structural changes in the hot corona, such as it was growing in size and becoming hotter with the decreasing accretion rate during the declining phase. The AGN is most likely evolving with a decreasing accretion rate toward a state similar to the low/hard state of black hole X-ray binaries.