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. We aimed 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. This
work is based around the analysis of u'BVr'i'z'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. 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 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. 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.
The final, definitive version of this article has been published in the Journal, Astronomy and Astrophysics, 637, 2020, © EDP Sciences.