Extreme AGN Variability in WISE: Powerful Flares and Candidate Tidal Disruption Events in AGN

Abstract

We present first results from a systematic study of extreme mid-infrared variability of active galaxies across the $>10$-year baseline of the Wide-field Infrared Survey Explorer (WISE) mission. WISE, which observed the full sky with a six-month cadence, is sensitive to transient events in both unobscured and obscured galaxies. This paper focuses on extreme flares in active galactic nuclei (AGNs). Previous studies identified flares such as tidal disruption events (TDEs) in inactive galaxies, typically from optical or X-ray surveys. However, flares are also expected in AGN and optical/X-ray searches are insensitive to events in dust-enshrouded AGN. In this pilot study, we present flares in two AGN identified from extreme mid-infrared color variability. One flare was also detected at optical wavelengths. While both events showed broadened H$\alpha$ emission several years after the flare peak, more recent spectroscopy reveals continued broadened emission only from the source with the optical flare. After considering several potential physical causes of the flares, including supernovae and micro-lensing, we suggest both events are TDEs in AGN. Intriguingly, the optical flare decays much faster than the expected $t^{-5/3}$ fallback rate expected for TDEs in quiescent galaxies, suggesting either a partial disruption or the tidal disruption of a star by an intermediate mass black hole embedded in the accretion disk of an AGN. This work demonstrates the power of WISE to identify the full census of TDEs, including events in dust-obscured AGN. Mid-infrared data probes the nature of the event and provides a bolometric measure of the flare energetics.