AT2025agpz in Rubin commissioning data: distinguishing a luminous interacting supernova from nuclear transients in compact galaxies
The Vera C. Rubin Observatory's Legacy Survey of Space and Time will discover unprecedented numbers of rare, long-lived optical transients, many of which will be too faint or too numerous for comprehensive spectroscopic follow-up. Characterising objects that span the boundaries between established transient classes is therefore essential for improving photometric classification. We present AT2025agpz, a luminous transient at $z=0.147$, discovered around peak by ATLAS but extensively monitored during Rubin commissioning observations of the Euclid Deep Field South. The transient is coincident with the nucleus of a faint dwarf galaxy, exhibits a rest-frame rise time of $77.9\pm1.3$ d, and reaches a peak bolometric luminosity of $6.3\times10^{43}$ erg s$^{-1}$. The exceptional depth and cadence of the Rubin commissioning data, combined with complementary DECam imaging, tightly constrain the explosion epoch and yield an early-time power-law rise of $n=3.33^{+0.33}_{-0.28}$. Follow-up spectroscopy reveals remarkably slow spectral evolution dominated by narrow Balmer emission, while high-resolution X-shooter observations resolve broad electron-scattering wings and multiple H$\alpha$ emission components characteristic of interaction-powered supernovae. Spectral energy distribution modelling indicates a low-mass, moderately star-forming host galaxy. Although AT2025agpz occupies observational parameter space shared by the recently identified Ambiguous Nuclear Transient population, its colour evolution, host environment, and emission-line morphology favour an interaction-powered luminous supernova. More broadly, this event highlights the growing observational overlap between luminous interacting supernovae and nuclear transients, illustrating both the challenges and opportunities for transient classification in the Rubin era.