  AT 2019abn was discovered in the nearby M51 galaxy by the Zwicky Transient
Facility at more than two magnitudes and around three weeks prior to its
optical peak. We aim to conduct a detailed photometric and spectroscopic
follow-up campaign for AT 2019abn, with early discovery allowing for
significant pre-maximum observations of an intermediate luminosity red
transient (ILRT) for the first time. This work is based on the analysis of
u'BVi'z'H photometry and low-resolution spectroscopy using the Liverpool
Telescope, medium-resolution spectroscopy with the Gran Telescopio Canarias
(GTC), and near-infrared imaging with the GTC and the Nordic Optical Telescope.
We present the most detailed optical light curve of an ILRT to date, with
multi-band photometry starting around three weeks before peak brightness. The
transient peaked at an observed absolute magnitude of M_r' = -13.1, although it
is subject to significant reddening from dust in M51, implying an intrinsic
M_r' ~ -15.2. The initial light curve showed a linear, achromatic rise in
magnitude before becoming bluer at peak. After peak brightness, the transient
gradually cooled. This is reflected in our spectra, which at later times show
absorption from such species as Fe I, Ni I and Li I. A spectrum taken around
peak brightness shows narrow, low-velocity absorption lines, which we interpret
as likely to originate from pre-existing circumstellar material. We conclude
that while there are some peculiarities, such as the radius evolution, AT
2019abn fits in well overall with the ILRT class of objects and is the most
luminous member of the class seen to date.
