A Broker Integrated Algorithm for Gravitational Wave - Electromagnetic Counterpart Searches in O4a and O4b Runs

Abstract

We present an automated framework to search for optical counterparts of LIGO-Virgo-KAGRA (LVK) gravitational wave (GW) superevents using public Zwicky Transient Facility (ZTF) alerts processed through the ALeRCE broker. The goal is to filter and identify optical transients potentially associated with binary black hole (BBH) mergers during the LVK O4a and O4b observing runs. Using the Automatic Learning for the Rapid Classification of Events (ALeRCE) infrastructure, we spatially query ZTF alerts within GW localization regions and apply machine learning classifiers, host-galaxy crossmatching, and temporal cuts within 200 days post-merger to isolate plausible candidates. Our search yielded one candidate in O4a and four in O4b, several consistent with the supernova or tidal disruption event regime. This work demonstrates that public alert brokers can establish a robust baseline for systematic searches for electromagnetic counterparts to GW superevents in current and future observing runs. Our algorithm provides a systematic approach to search for BBH counterparts for all significant LVK GW superevents using survey telescope alerts. The search, together with the accompanying analysis, demonstrates the significance of the counterpart candidates, with one candidate ultimately identified as a transient event consistent with a Bowen fluorescence flare in a now discarded active galactic nucleus (AGN).