AT2021yky: A Fast-Rising Optical Transient with Evolving Broad Hydrogen Emission Consistent with an Ambiguous Nuclear Transient

Nuclear transients are powerful probes of supermassive black hole properties, offering insight into black hole mass, accretion physics, and the structure of galactic nuclei. Among these, a growing class of events cannot be classified as either tidal disruption events (TDEs) or active galactic nuclei (AGN) flares, and their physical origins remain poorly understood. We present a multi-wavelength photometric and spectroscopic analysis of AT2021yky (ZTF21abzciqh), an ambiguous nuclear transient (ANT) at a redshift of z = 0.076. AT2021yky reached a peak bolometric luminosity of L peak = (4.1 plus or minus 1.1) times 10 43 erg s-1, with a rise-time of 18.2 plus or minus 0.7 days. The early-time UV/optical emission is well described by a blackbody with a temperature of T 1.4 times 10 4 K, cooler than most optically selected TDEs. No X-ray emission from the transient is detected, with a 3 sigma limit of L X 3.4 times 10 41 erg s-1 near peak. Spectroscopic observations reveal a largely featureless blue continuum with broad (FWHM about 11,000 km s-1) H alpha emission line that appears around 20 - 40 days post-peak. The host-galaxy emission-line ratios indicate the presence of an AGN, though the absence of optical or mid-IR variability and a non-AGN mid-IR color suggest it is weak. AT2021yky exhibits a rapid rise time comparable to that of luminous fast blue optical transients (LFBOTs), while its decay timescale and late-time broad H alpha emission resemble those observed in TDEs. However, its cooler blackbody temperature and the absence of He II and Balmer emission lines other than H alpha instead favour its classification as an ANT.