Gaia21bty: An EXor light curve exhibiting a FUor spectrum

Gaia21bty, a pre-main sequence star that previously had shown aperiodic dips in its light curve, underwent a considerable Delta G~2.9 mag brightening that occurred over a few months between 2020 October - 2021 February. The Gaia lightcurve shows that the star remained near maximum brightness for about 4-6 months, and then started slowly fading over the next 2 years, with at least three superimposed ~1 mag sudden rebrightening events. Whereas the amplitude and duration of the maximum is typical for EXors, optical and near-infrared spectra obtained at the maximum are dominated by features which are typical for FUors. Modelling of the accretion disc at the maximum indicates that the disc bolometric luminosity is 43 Lsun and the mass accretion rate is 2.5 x 10^{-5} Msun/yr, which are typical values for FUors even considering the large uncertainty in the distance (1.7_{-0.4}^{+0.8} kpc). Further monitoring is necessary to understand the cause of the quick brightness decline, the rebrightening, and the other post-outburst light changes, as our multi-colour photometric data suggest that they could be caused by a long and discontinuous obscuration event. We speculate that the outburst might have induced large-scale inhomogeneous dust condensations in the line of sight leading to such phenomena, whilst the FUor outburst continues behind the opaque screen.