A decades-long fast-rise-exponential-decay flare in low-luminosity AGN NGC 7213

Abstract

We analysed the four-decades-long X-ray light curve of the low-luminosity active galactic nucleus (LLAGN) NGC 7213 and discovered a fast-rise-exponential-decay (FRED) pattern, i.e. the X-ray luminosity increased by a factor of $\approx 4$ within 200d, and then decreased exponentially with an $e$-folding time $\approx 8116$d ($\approx 22.2$ yr). For the theoretical understanding of the observations, we examined three variability models proposed in the literature: the thermal-viscous disc instability model, the radiation pressure instability model, and the tidal disruption event (TDE) model. We find that a delayed tidal disruption of a main-sequence star is most favourable; either the thermal-viscous disk instability model or radiation pressure instability model fails to explain some key properties observed, thus we argue them unlikely.

Accepted for publication in MNRAS, updated version after proof corrections

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• The Galaxy Activity, Torus and Outflow Survey (GATOS) I. ALMA images of dusty molecular tori in Seyfert galaxies
• Evidence for A Hot Wind from High-resolution X-ray Spectroscopic Observation of the Low-luminosity Active Galactic Nucleus in NGC 7213
• An Explanation for Overrepresentation of Tidal Disruption Events in Post-starburst Galaxies
• Fast giant flares in discs around supermassive black holes