2024ApJ...975..140L

Time-domain studies of active galactic nuclei (AGNs) offer a powerful tool for understanding black hole accretion physics. Prior to the optical outburst on 2017 December 23, 1ES 1927+654 was classified as a “true” type 2 AGN, an unobscured source intrinsically devoid of broad-line emission in polarized spectra. Through our 3 yr monitoring campaign spanning X-ray to ultraviolet/optical wavelengths, we analyze the post-outburst evolution of the spectral energy distribution (SED) of 1ES 1927+654. Examination of the intrinsic SED and subsequent modeling using different models reveal that the post-outburst spectrum is best described by a combination of a disk, blackbody, and corona components. We detect systematic SED variability and identify four distinct stages in the evolution of these components. During the event the accretion rate is typically above the Eddington limit. The correlation between ultraviolet luminosity and optical to X-ray slope (α_OX) resembles that seen in previous studies of type 1 AGNs, yet exhibits two distinct branches with opposite slopes. The optical bolometric correction factor (κ₅₁₀₀) is ∼10 times higher than typical AGNs, again displaying two distinct branches. Correlations among the corona optical depth, disk surface density, and α_OX provide compelling evidence of a disk-corona connection. The X-ray corona showcases systematic variation in the compactness-temperature plot. Between 200 and 650 days, the corona is “hotter when brighter,” whereas after 650 days, it becomes “cooler when brighter.” This bimodal behavior, in conjunction with the bifurcated branches of α_OX and κ₅₁₀₀, offers strong evidence of a transition from a slim disk to a thin disk ∼650 days after the outburst.