Abstract Changing-look transitions challenge our understanding of active galactic nuclei (AGN), exhibiting dramatic changes in broad-line emission and continuum flux on timescales of months to years. We present a detailed study of the spectroscopically confirmed changing-look AGN ZTF18abuamgo. Combining photometric survey data with spectroscopy spanning three epochs over 20 years, we identify a turn-on transition from a Type 1.5 to Type 1.2 AGN and estimate the timescale of this change to be as short as four years. Spectral analysis indicates that this transformation is driven by a rapid increase in accretion rate, with the Eddington ratio rising from 0.032 ± 0.005 in the dim state to 0.08 ± 0.01 in the bright state. For the first time in a changing-look AGN, we apply the Boltzmann plot method to the visible Balmer series emission, deriving broad line region electron temperatures of 11, 800 ± 900 K and 11, 900 ± 2, 400 K in 2022 and 2024, respectively. Applying single-epoch black hole mass estimation to the brightening Hα emission, we find a mass of (5.0 ± 0.4) × 107M⊙. The consistency in this estimate across all spectroscopic epochs suggest that even highly variable broad lines in CL-AGN do not bias the results derived using this method. Our results demonstrate that objects like ZTF18abuamgo provide a unique laboratory to study extreme AGN variability, probe the physical conditions in the broad line region, and assess the limitations of widely used black hole mass estimation methods.
Carpenter et al. (2026) studied this question.