Context : AT 2019abn was discovered in the nearby M51 galaxy , by the Zwicky Transient Facility more than two magnitudes , and around 3 weeks , prior to its optical peak .

Aims : To conduct a detailed photometric and spectroscopic follow-up campaign for AT 2019abn , with the early discovery allowing significant pre-maximum observations of an intermediate luminosity red transient ( ILRT ) for the first time .

Methods : This work is based around the analysis of u ^ { \prime } BVr ^ { \prime } i ^ { \prime } z ^ { \prime } H photometry and low-resolution spectroscopy with the Liverpool Telescope , medium-resolution spectroscopy with Gran Telescopio Canarias ( GTC ) and near-infrared imaging with GTC and the Nordic Optical Telescope .

Results : 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 } \sim - 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 species such 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 originating from pre-existing circumstellar material .

Conclusions : We conclude that , while there are some peculiarities , such as the radius evolution , AT 2019abn overall fits in well with the ILRT class of objects , and is the most luminous member of the class seen to date .