Tracing Active Galactic Nuclei Properties Through a Changing-look Event

TL;DR

This study investigates a changing-look AGN, revealing rapid accretion rate changes, electron temperature measurements, and consistent black hole mass estimates over multiple epochs, enhancing understanding of AGN variability.

Contribution

First application of the Boltzmann plot method to a changing-look AGN, linking spectral changes to physical conditions and black hole mass estimation.

Findings

Turn-on transition from Type 1.5 to Type 1.2 AGN within four years.

Eddington ratio increased from 0.032 to 0.08 during the transition.

Broad line region electron temperatures measured at ~12,000 K.

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 $0.032 \pm 0.005$ in the dim state to $0.08 \pm 0.01$ 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 $11,800 \pm 900$ K and $11,900 \pm 2,400$ K in 2022 and 2024, respectively. Applying single-epoch black hole mass estimation to the brightening H$\alpha$ emission, we find a mass of $(5.0 \pm 0.4) \times 10^7 M_\odot$. 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.