| Title: |
AT2025ulz and S250818k: Zooming in with the Hubble Space Telescope |
| Authors: |
Yu-Han Yang; Eleonora Troja; Marko Ristić; Muskan Yadav; Massine El Kabir; Rubén Sánchez-Ramírez; Rosa L. Becerra; Chris L. Fryer; Brendan O’Connor; Simone Dichiara; Alberto J. Castro-Tirado; Camila Angulo-Valdez; Josefa Becerra González; José A. Font; Ori Fox; Lei Hu; Youdong Hu; William H. Lee; Margarita Pereyra; Alicia M. Sintes; Alan M. Watson; K. Océlotl C. López Mendoza |
| Source: |
The Astrophysical Journal Letters, Vol 996, Iss 2, p L24 (2026) |
| Publisher Information: |
IOP Publishing, 2026. |
| Publication Year: |
2026 |
| Collection: |
LCC:Astrophysics |
| Subject Terms: |
Time domain astronomy; Gravitational waves; Gamma-ray bursts; Compact objects; Neutron stars; Supernovae; Astrophysics; QB460-466 |
| Description: |
AT2025ulz is an optical/near-infrared transient discovered during follow-up of the candidate gravitational wave (GW) event S250818k. Its young age (≲1 day), 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 and the Hubble Space Telescope (HST). Although the early time evolution of AT2025ulz resembles some aspects of a kilonova, its rapid onset (∼3 hr after the GW trigger) and luminosity (a factor of ∼5 brighter than AT2017gfo in g band) are difficult to reproduce. Only a small subset of our kilonova models matches its multicolor light curve, and the inferred ejecta mass is uncomfortably large given the low chirp mass (≲0.87 M _⊙ ) of the GW candidate. HST observations place the transient within a nearby ( z = 0.08489) spiral galaxy with ongoing star formation and measure a color (F336W − F160W ≈ 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. |
| Document Type: |
article |
| File Description: |
electronic resource |
| Language: |
English |
| ISSN: |
2041-8205 |
| Relation: |
https://doaj.org/toc/2041-8205 |
| DOI: |
10.3847/2041-8213/ae2dea |
| Access URL: |
https://doaj.org/article/8a0c19a9fc074954bb220ac4ee6ca9f4 |
| Accession Number: |
edsdoj.8a0c19a9fc074954bb220ac4ee6ca9f4 |
| Database: |
Directory of Open Access Journals |