OASIS: On-the-fly ASynchronous Image Stream classification -- Transformer-based models for the early detection of strongly lensed supernovae

Abstract

Time-domain surveys often produce asynchronous image sequences across irregular observation times and photometric bands. In many applications, predictions are needed after each new observation, particularly when transient sources must be identified early to trigger follow-up. Strongly lensed supernovae (LSNe), for which follow-up is particularly time-critical, provide a useful example. We formulate this setting as On-the-fly ASynchronous Image Stream (OASIS) classification and explore reusable transformer architectures for causal, on-the-fly classification. We demonstrate these architectures on the lensed-transient classification problem studied by Bag et al. (2026a,b), distinguishing LSNe Ia from unlensed astrophysical transients. We compare them with the recurrent baseline adopted by Bag et al. (2026b), which combines a ConvLSTM branch for image processing with a separate LSTM branch for encoding observation times. The baseline maps asynchronous observations onto a fixed band-by-observation grid, filling unobserved bands with zeros. Using a common training setup, we compare two transformer-based approaches to modeling interactions among spatio-temporal tokens: factorized attention, which separates spatial and temporal interactions, and joint attention, which models them together. On the demonstration dataset, factorized architectures achieve mean losses of $0.095$--$0.100$, compared with $0.128$ for the recurrent baseline and $0.107$ or higher for joint architectures. A selective readout further reduces the best loss to $0.089$ for a factorized model. These results suggest that factorized attention suits this dataset better, without implying universal architectural superiority. We release the architectures as reusable foundations for time-domain astronomy and other applications involving evolving spatial features that require on-the-fly predictions.