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

TL;DR

This study analyzes four decades of X-ray data from the low-luminosity AGN NGC 7213, revealing a significant flare with a rapid rise and exponential decay, and suggests a delayed tidal disruption event as the most plausible cause.

Contribution

It provides the first detailed analysis of a long-term X-ray flare in a LLAGN and proposes a tidal disruption event as the likely mechanism, challenging existing instability models.

Findings

Identified a fast-rise-exponential-decay pattern in X-ray luminosity.

Disfavored thermal-viscous and radiation pressure instability models.

Supported delayed tidal disruption of a star as the flare's cause.

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.