AT2025ulz and S250818k: Investigating early time observations of a subsolar mass gravitational-wave binary neutron star merger candidate

TL;DR

This paper reports on early observations of a candidate electromagnetic counterpart to a low-probability gravitational-wave neutron star merger, highlighting the challenges in distinguishing kilonovae from other transients with limited early data.

Contribution

It provides a detailed photometric and spectroscopic analysis of AT2025ulz, demonstrating the ambiguity in early data and emphasizing the need for extensive follow-up to confirm kilonovae.

Findings

Early data are ambiguous between kilonova and shock cooling models.

Extended monitoring revealed a supernova-like evolution.

Identifying BNS merger counterparts requires significant observational effort.

Abstract

Over the past LIGO--Virgo--KAGRA (LVK) observing runs, it has become increasingly clear that identifying the next electromagnetic counterparts to gravitational-wave (GW) neutron star mergers will likely be more challenging compared to the case of GW170817. The rarity of these GW events, and their electromagnetic counterparts, motivates rapid searches of any candidate binary neutron star (BNS) merger detected by the LVK. We present our extensive photometric and spectroscopic campaign of the candidate counterpart AT2025ulz to the low-significance GW event S250818k, which had a ${\sim} 29\%$ probability of being a BNS merger. We demonstrate that during the first five days, the luminosity and color evolution of AT2025ulz are consistent with both kilonova and shock cooling models, although a Bayesian model comparison shows preference for the shock cooling model, underscoring the ambiguity inherent to early data obtained over only a few days. Continued monitoring beyond this window reveals a rise and color evolution incompatible with kilonova models and instead consistent with a supernova. This event emphasizes the difficulty in identifying the electromagnetic counterparts to BNS mergers and the significant allotment of observing time necessary to robustly differentiate kilonovae from impostors.