The promise of self-supervised and active learning for Strong Lens discovery: Astronomaly applied to KiDS

Paper abstract Strong gravitational lenses (SGLs) are rare systems whose discovery currently relies primarily on supervised machine learning methods trained on large simulated datasets. We present the first application of Astronomaly:PROTEGE to SGL discovery, demonstrating that a human-in-the-loop active learning framework can efficiently identify lenses in large imaging surveys without the need for simulated training data. We consider a sample of 3.7 million bright galaxies from the Kilo-Degree Survey (KiDS) DR4. Feature representations are extracted using a convolutional neural network pre-trained on the ImageNet dataset and subsequently fine-tuned on KiDS data using the self-supervised Bootstrap Your Own Latent (BYOL) framework. Within the embedding of these representations, the active learning loop of Astronomaly iteratively selects the most informative systems for expert inspection. A total of 3,000 objects are inspected across multiple rounds, yielding 34 high-quality (grade A/B) SGL candidates. On the basis that these systems occupy similar regions in the learned feature space, we expand this sample through nearest-neighbour similarity analysis. Including the active learning discoveries, we identify a total of 140 grade A/B candidates and more than 1,000 additional lower-confidence systems (grade C). Among the A/B candidates, 81 are newly identified, while 22% of previously known grade A/B KiDS lenses are recovered. These results demonstrate strong potential for next-generation surveys such as Euclid, Roman, and Rubin's Legacy Survey of Space and Time. With approximately 60% of the high-quality candidates newly reported, this approach complements supervised methods by reducing reliance on simulations and enabling the discovery of a diverse population of SGLs. A paper describing this work is in preparation; this record will be updated with a link once it is available. About this dataset All candidates in this dataset have been visually inspected by three experts, and assigned a confidence grade based on their likelihood of being a SGL. The catalogue contains 1,172 candidates in total: 8 grade A, 132 grade B, and 1,032 grade C. Non-lens systems (grade X) are not included in this release. The table is provided as a single file, sorted by grade and then by Right Ascension. Grading scheme Grade A: high-confidence strong lens candidates Grade B: likely lens candidates requiring further confirmation Grade C: low-confidence candidates showing some lens-like feature Columns KiDS_ID: KiDS unique object identifier RA: Right Ascension (degrees) DEC: Declination (degrees) Grade: Expert-assigned confidence grade (A, B, or C) Method: Stage(s) through which the candidate was identified (AL, NN, or AL+NN) Code The wrapper code used for this work is available at https://github.com/margres/KiDS_lenses_astronomaly_protege