AT2025ulz and S250818k: zooming in with the Hubble Space Telescope

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.

💡 Research Summary

AT2025ulz was discovered as a rapidly fading optical/near‑infrared transient within the localization region of the sub‑threshold gravitational‑wave candidate S250818k. The GW event, reported by LIGO‑Virgo‑KAGRA, had a large sky area (≈ 786 deg²) and a distance of 259 ± 74 Mpc, with a modest probability (≈ 29 %) of being a binary neutron‑star merger. Because the transient appeared only ~3 hours after the GW trigger, showed a very fast decline, and exhibited strong color evolution over the first two days, it was initially flagged as a kilonova candidate.

The authors carried out an extensive follow‑up campaign using the Gran Telescopio Canarias (GTC) and the Hubble Space Telescope (HST). GTC imaging in g, r, i, z bands captured the early light curve, while low‑resolution spectroscopy (R ≈ 1000) provided redshift confirmation (z = 0.08489) and identified broad absorption features. HST observations (F336W, F606W, F110W, F160W) were obtained at 4.8 days and 8.5 days post‑trigger, allowing precise photometry after careful host‑galaxy subtraction with GALFIT.

Key observational findings:

1. The early g‑band luminosity of AT2025ulz is ≈ 1.5 mag (≈ 5×) brighter than AT2017gfo at comparable epochs, far exceeding expectations for a kilonova.
2. Only a tiny subset of kilonova models can reproduce the multi‑color light curve, and those require ejecta masses that are implausibly large given the low chirp mass (≤ 0.87 M⊙) inferred for S250818k.
3. The HST color F336W – F160W ≈ 1.4 mag is dramatically bluer than the ≈ 7 mag color of AT2017gfo at a similar phase, indicating a hotter photosphere inconsistent with kilonova cooling.
4. Spectra show strong H α and H β emission, along with a broad P‑Cygni absorption centered near 6750 Å, corresponding to an expansion velocity of ≈ 15,400 km s⁻¹—characteristic of Type II supernovae.
5. The host galaxy is a star‑forming spiral (Sérsic index n ≈ 1–2) rather than a quiescent elliptical, further supporting a core‑collapse supernova origin.

The authors therefore reclassify AT2025ulz as a Type II supernova that experienced an early shock‑cooling phase followed by a normal photospheric phase. The rising r‑band flux observed up to ≈ 13 days, the blue UV‑optical color, and the high luminosity all align with this interpretation.

This work highlights the challenges of associating optical transients with low‑significance GW triggers. It demonstrates that early‑time color evolution alone can be misleading, as core‑collapse supernovae can mimic kilonova signatures in the first few days. Robust classification requires a combination of multi‑band photometry, high‑resolution imaging to isolate the transient from its host, and timely spectroscopy. The study underscores the importance of rapid, coordinated follow‑up across facilities to avoid false associations and to correctly identify the astrophysical nature of GW counterparts.