# A new class of flares from accreting supermassive black holes

## TL;DR

This paper reports the discovery of a new class of flares from accreting supermassive black holes characterized by sustained ultraviolet-optical emission increases, spectral features including Bowen fluorescence, and implications for understanding SMBH accretion variability.

## Contribution

The study introduces a new class of SMBH-related flares with unique spectral and temporal properties, expanding the understanding of accretion variability phenomena.

## Key findings

- Identification of a new SMBH flare class with sustained emission.
- Spectral features include Bowen fluorescence linked to high-velocity gas.
- Flares show little evolution over 14 months, indicating prolonged accretion events.

## Abstract

Accreting supermassive black holes (SMBHs) can exhibit variable emission across the electromagnetic spectrum and over a broad range of timescales. The variability of active galactic nuclei (AGNs) in the ultraviolet and optical is usually at the few tens of per cent level over timescales of hours to weeks. Recently, rare, more dramatic changes to the emission from accreting SMBHs have been observed, including tidal disruption events, 'changing look' AGNs and other extreme variability objects. The physics behind the 're-ignition', enhancement and 'shut-down' of accretion onto SMBHs is not entirely understood. Here we present a rapid increase in ultraviolet-optical emission in the centre of a nearby galaxy, marking the onset of sudden increased accretion onto a SMBH. The optical spectrum of this flare, dubbed AT 2017bgt, exhibits a mix of emission features. Some are typical of luminous, unobscured AGNs, but others are likely driven by Bowen fluorescence - robustly linked here with high-velocity gas in the vicinity of the accreting SMBH. The spectral features and increased ultraviolet flux show little evolution over a period of at least 14 months. This disfavours the tidal disruption of a star as their origin, and instead suggests a longer-term event of intensified accretion. Together with two other recently reported events with similar properties, we define a new class of SMBH-related flares. This has important implications for the classification of different types of enhanced accretion onto SMBHs.