Kavli Affiliate: Daniel R. Wilkins
| Summary:
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 $L_rm peak = (4.1 pm 1.1) times 10^43~mathrmerg~s^-1$, with a rise-time of $18.2 pm 0.7$ days. The early-time UV/optical emission is well described by a blackbody with a temperature of $T simeq 1.4 times 10^4$ K, cooler than most optically selected TDEs. No X-ray emission from the transient is detected, with a $3σ$ limit of $L_X lesssim 3.4 times 10^41$ erg s$^-1$ near peak. Spectroscopic observations reveal a largely featureless blue continuum with broad (FWHM$sim 11,000$ km s$^-1$) H$α$ emission line that appears around 20$-$40 days post-peak. The host-galaxy emission-line ratios indicate the presence of an AGN, though the absence of optical or mid-IR variability and a non-AGN mid-IR color suggest it is weak. AT2021yky exhibits a rapid rise time comparable to that of luminous fast blue optical transients (LFBOTs), while its decay timescale and late-time broad H$α$ emission resemble those observed in TDEs. However, its cooler blackbody temperature and the absence of He II and Balmer emission lines other than H$α$ instead favour its classification as an ANT.
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