  We report on the discovery and follow-up of a peculiar transient, OGLE17aaj,
which occurred in the nucleus of a weakly active galaxy. We investigate whether
it can be interpreted as a new candidate for a tidal disruption event (TDE). We
present the OGLE-IV light curve that covers the slow 60-day-long rise to
maximum along with photometric, spectroscopic, and X-ray follow-up during the
first year. OGLE17aaj is a nuclear transient exhibiting some properties similar
to previously found TDEs, including a long rise time, lack of
colour-temperature evolution, and high black-body temperature. On the other
hand, its narrow emission lines and slow post-peak evolution are different from
previously observed TDEs. Its spectrum and light-curve evolution is similar to
F01004-2237 and AT 2017bgt. Signatures of historical low-level nuclear
variability suggest that OGLE17aaj may instead be related to a new type of
accretion event in active super-massive black holes.
