2021ApJ...907L..29H

Understanding the origin of Fe II emission is important because it is crucial to construct the main sequence of active galactic nuclei (AGNs). Despite several decades of observational and theoretical effort, the location of the optical iron emitting region and the mechanism responsible for the positive correlation between the Fe II strength and the black hole accretion rate remain open questions. In this Letter, we report the optical Fe II response to the central outburst in PS1-10adi, a candidate tidal disruption event taking place in an AGN at z = 0.203 that has aroused extensive attention. For the first time, we observe that the Fe II response in the rising phase of its central luminosity is significantly more prominent than that in the decline phase, showing a hysteresis effect. We interpret this hysteresis effect as a consequence of the gradual sublimation of the dust grains situating at the inner surface of the torus into gas when the luminosity of the central engine increases. It is the iron element released from the sublimated dust that contributes evidently to the observed Fe II emission. This interpretation, together with the weak response of the Hβ emission as we observe, naturally explains the applicability of relative Fe II strength as a tracer of the Eddington ratio. In addition, optical iron emission of this origin renders the Fe II time lag a potential "standard candle" with cosmological implications.