A broker-integrated algorithm for electromagnetic counterpart searches to gravitational-wave events in O4a and O4b runs

Context. We present an automated framework for a search for optical counterparts of LIGO–Virgo–KAGRA (LVK) gravitational-wave (GW) superevents using public Zwicky Transient Facility (ZTF) alerts processed through the Automatic Learning for the Rapid Classification of Events (ALeRCE) broker. Aims. The goal is to filter and identify optical transients that might be associated with binary black hole (BBH) mergers during the LVK O4a and O4b observing runs. Methods. Using the infrastructure called ALeRCE, we spatially queried ZTF alerts within GW localization regions and applied machine-learning classifiers, host-galaxy cross-matching, and temporal cuts within 200 days post merger to isolate plausible candidates. Results. Our search yielded one candidate in O4a and four in O4b, several of which are consistent with the supernova (SN) – tidal disruption event (TDE) regime. This work demonstrates that public-alert brokers can set a baseline for performing systematic searches for electromagnetic counterparts of GW superevents in current and future observing runs. Conclusions. Our algorithm provides a way to search for the BBH counterparts for all significant LVK GW superevents using survey telescope alerts. The search and the accompanying analysis demonstrate the significance of the counterpart candidates. One candidate, consistent with a Bowen fluorescence flare in an active galactic nucleus, is ultimately disfavored as a counterpart.