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

¹ Departamento de Astronomia (DAS), Universidad de Chile, Camino el Observatorio 1515, Las Condes, Santiago, Chile
² Data and Artificial Intelligence Initiative (IDIA), Faculty of Physical and Mathematical Sciences, Universidad de Chile, Santiago, Chile
³ Centro de Excelencia en Astrofisica y Technologia Afines, Av. Vicuña Mackenna 4860, San Joaquín, Santiago, Chile
⁴ Millennium Institute of Astrophysics, Nuncio Monseñor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile
⁵ Center for Mathematical Modeling, Universidad de Chile, Beauchef 851, Santiago 8370456, Chile
⁶ Physik-Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
⁷ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860 Macul Santiago, 7820436, Chile
⁸ Centro de Astronomia, Universidad de Antofogasta, 02800 Antofagasta, Chile
⁹ UKIRT Observatory, Institute for Astronomy, 640 N. A’ohoku Place, University Park, Hilo, Hawai’i, 96720, USA
¹⁰ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441 Santiago, Chile
¹¹ Centro Interdisciplinario de Data Science, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441 Santiago, Chile
¹² Astronomy Department, Universidad de Concepción, Casilla 160-C, 4030000 Concepción, Chile
¹³ Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes (TITANs), Concepción, Chile

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 December 2025
Accepted: 9 March 2026

Abstract

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.