  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.
