AT2025ulz and S250818k: zooming in with the Hubble Space Telescope

TL;DR

This study uses Hubble Space Telescope observations to analyze AT2025ulz, a transient initially suspected as a kilonova, but evidence suggests it is a supernova with shock-cooling features, challenging its initial classification.

Contribution

The paper provides detailed multi-wavelength observations of AT2025ulz, demonstrating its classification as a supernova rather than a kilonova, and discusses implications for transient identification.

Findings

AT2025ulz is located in a star-forming galaxy at z=0.08489.

Its color and luminosity are inconsistent with kilonova models.

Spectroscopy reveals broad absorption features characteristic of supernovae.

Abstract

AT2025ulz is an optical/near-infrared transient discovered during follow-up of the candidate gravitational wave (GW) event S250818k. Its young age ($\lesssim$1 d), rapid decline and strong color evolution over the first 48 hr classify it as a potential kilonova candidate. In this work, we present the results of our observing campaign, carried out with the Gran Telescopio Canarias (GTC) and the Hubble Space Telescope (HST). Although the early time evolution of AT2025ulz resembles some aspects of a kilonova, its rapid onset ($\sim$3 hr after the GW trigger) and luminosity (a factor of $\sim5$ brighter than AT2017gfo in $g$-band) are difficult to reproduce. Only a small subset of our kilonova models matches its multi-color light curve, and the inferred ejecta mass is uncomfortably large given the low chirp mass ($\lesssim\!0.87\!$ M$_{\odot}$) of the GW candidate. HST observations place the transient within a nearby ($z=0.08489$) spiral galaxy with on-going star-formation and measure a color ($F336W-F160W\!\approx\!1.4$ mag) that is too blue to match with a kilonova. Our data support the classification of AT2025ulz as a supernova, initially undergoing a shock-cooling phase and later entering its photospheric phase, and spectroscopically identified via its broad absorption features.