A Hybrid Framework for Kilonova Anomaly Detection Using Single-epoch SEDs from the 7-Dimensional Telescope

We develop a hybrid framework to identify kilonovae (KNe), using single-epoch, medium-band spectral energy distributions from the 7-Dimensional Telescope (7DT). The framework integrates an unsupervised anomaly classifier (Isolation Forest) to flag unusual events with a supervised multiclass classifier (XGBoost) that characterizes eight common transient types. Trained on realistically simulated 7DT photometry accounting for per-filter sensitivity, the classifier achieves macro F1 ∼ 0.80 (∼0.82) with 20 (40) filters across eight classes, Type Ia/Ibc/II SNe, SLSNe, TDEs, AGN, stellar variables, and asteroids. Without direct training, the anomaly detector recovers >90% of simulated and observed optically detectable KNe (AT 2017gfo) with a low contamination fraction, with a caveat of limitations of the training sample such as limited redshift range of SNe (z < 0.15), and a relatively small number of early non-KNe spectra. A SHAP-based feature analysis reveals that only ∼40%–50% of the most informative filters are sufficient to retain near-baseline performance, while red-end filters contribute little. Combining the top-ranked half of the 40 7DT filters with a single LSST band reproduces the full-model accuracy within 1%–2%, suggesting practical follow-up strategies. These results demonstrate that 7DT’s medium-band system enables rapid, interpretable classifications and reliable anomaly alerts from single-epoch data—promising for gravitational-wave follow-up, Rubin alert stream filtering, and serendipitous transient discovery in the 7DT survey.