An Extraordinary Response of Iron Emission to the Central Outburst in a Tidal Disruption Event Candidate

TL;DR

This study observes a unique hysteresis in iron emission response during a tidal disruption event, linking dust sublimation to iron emission and suggesting new uses for eii\ as a cosmic probe.

Contribution

First detection of hysteresis in eii\ emission response during a TDE, proposing dust sublimation as the mechanism behind iron emission variability.

Findings

eii\ response is stronger during luminosity rise than decline.

Dust sublimation explains the hysteresis effect in eii\ emission.

eii\ emission can serve as a standard candle for cosmology.

Abstract

Understanding the origin of \feii\ 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 \feii\ strength and the black hole accretion rate remain open questions as yet. In this letter, we report the optical \feii\ response to the central outburst in PS1-10adi, a candidate tidal disruption event (TDE) taking place in an AGN at $z = 0.203$ that has aroused extensive attention. For the first time, we observe that the \feii\ 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 \feii\ emission. This interpretation, together with the weak response of the \hb\ emission as we observe, naturally explains the applicability of relative \feii\ strength as a tracer of the Eddington ratio. In addition, optical iron emission of this origin renders the \feii\ time lag a potential "standard candle" with cosmological implications.