A Two-zone Accretion Disk in the Changing-look Active Galactic Nucleus 1ES 1927+654: Physical Implications for Tidal Disruption Events and Super-Eddington Accretion

Abstract

The properties of slim accretion disks, while crucial for our understanding of black hole growth, have yet to be studied extensively observationally. We analyze the multi-epoch broad-band spectral energy distribution of the changing-look active galactic nucleus 1ES 1927+654 to derive the properties of its complex, time-dependent accretion flow. The accretion rate decays as $\dot{M} \propto t^{-1.53}$, consistent with the tidal disruption of a $1.1\, M_\odot$ star. Three components contribute to the spectral energy distribution: a central overheated zone resembling a slim disk, an outer truncated thin disk, and a hot corona. Photon trapping in the slim disk triggered by the high initial $\dot{M}$ was characterized by a low radiation efficiency ($3\%$), which later more than doubled ($8\%$) after $\dot{M}$ dropped sufficiently low for the disk to transition to a geometrically thin state. The blackbody temperature profile $T \propto R^{-0.60}$ for the inner overheated zone matches the theoretical expectations of a slim disk, while the effective temperature profile of $T \propto R^{-0.69}$ for the outer zone is consistent with the predictions of a thin disk. Both profiles flatten toward the inner boundary of the disk as a result of Compton cooling in the corona. Our work presents compelling observational evidence for the existence of slim accretion disks and elucidates the key parameters governing their behavior, paving the way for further exploration in this area.