Optimizing Kilonova Searches: A Case Study of the Type IIb SN 2025ulz in the Localization Volume of the Low-significance Gravitational Wave Event S250818k

Abstract Kilonovae, the ultraviolet/optical/infrared counterparts to binary neutron star mergers, are an exceptionally rare class of transients. Optical follow-up campaigns are plagued by contaminating transients, which may mimic kilonovae but do not receive sufficient observations to measure the full photometric evolution. In this work, we present an analysis of the multiwavelength dataset of supernova (SN) 2025ulz, a proposed kilonova candidate following the low-significance detection of gravitational waves originating from the potential binary neutron star merger S250818k. Despite an early rapid decline in brightness, our multiwavelength observations of SN 2025ulz reveal that it is a type IIb SN. As part of this analysis, we demonstrate the capabilities of a novel quantitative scoring algorithm to determine the likelihood that a transient candidate is a kilonova, based primarily on its three-dimensional location and light-curve evolution. We also apply our scoring algorithm to other transient candidates in the localization volume of S250818k and find that, at all times after the discovery of SN 2025ulz, there are ≥4 candidates with a score comparable to SN 2025ulz, indicating that the kilonova search may have benefited from the additional follow-up of other candidates. During future kilonova searches, this type of scoring algorithm will be useful to rule out contaminating transients in real time, optimizing the use of valuable telescope resources.

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Publication Details

Journal
The Astrophysical Journal Letters
Published
2025-11-25
DOI
https://doi.org/10.3847/2041-8213/ae17a8
Citations
3
Primary Topic
Gamma-ray bursts and supernovae
Type
article
Field-Weighted Citation Impact
3.46
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article

Optimizing Kilonova Searches: A Case Study of the Type IIb SN 2025ulz in the Localization Volume of the Low-significance Gravitational Wave Event S250818k

C. D. Kilpatrick, C. L. Ransome, Walter W. Golay, D. Reichart et al.
3 citations
The Astrophysical Journal Letters
Gamma-ray bursts and supernovae
3.46
article

Optimizing Kilonova Searches: A Case Study of the Type IIb SN 2025ulz in the Localization Volume of the Low-significance Gravitational Wave Event S250818k

C. D. Kilpatrick, C. L. Ransome, Walter W. Golay, D. Reichart, M. Lundquist, Mathieu Renzo, N. Butler, Jay Strader, Wen‐fai Fong, Nathan Smith, Philip N. Daly, Garrett K. Keating, Jeniveve Pearson, Collin T. Christy, Nicholas Vieira, Sanchayeeta Borthakur, K. D. Alexander, Jennifer E. Andrews, T. Laskar, D. Hiramatsu, Tarraneh Eftekhari, G. Hosseinzadeh, Ramprasad Rao, Benjamin J. Weiner, Noah Franz, P.-Y. Noël, David E. Trilling, Harsh Kumar, Kristine Spekkens, Genevieve Schroeder, Paul C. Duffell, Alexander Gagliano, Shelbi Hostler Schimpf, Kathryne J. Daniel, Clécio R. Bom, David J. Sand, D. L. Coppejans, E. Berger, Angelo Santos, Bhagya Subrayan, Mark Gurwell, Peter K. G. Williams, Brian Hsu, Aldana Grichener, L. Santana-Silva, Jillian Rastinejad, K. Paterson, Erika Hamden, Carl Fields, Ramya M. Anche, Manisha Shrestha, R. Margutti, Vasileios Paschalidis, Vikram Manikantan, P. Darc, K. Azalee Bostroem, Kali Salmas
article en
3 citations

Abstract

Abstract Kilonovae, the ultraviolet/optical/infrared counterparts to binary neutron star mergers, are an exceptionally rare class of transients. Optical follow-up campaigns are plagued by contaminating transients, which may mimic kilonovae but do not receive sufficient observations to measure the full photometric evolution. In this work, we present an analysis of the multiwavelength dataset of supernova (SN) 2025ulz, a proposed kilonova candidate following the low-significance detection of gravitational waves originating from the potential binary neutron star merger S250818k. Despite an early rapid decline in brightness, our multiwavelength observations of SN 2025ulz reveal that it is a type IIb SN. As part of this analysis, we demonstrate the capabilities of a novel quantitative scoring algorithm to determine the likelihood that a transient candidate is a kilonova, based primarily on its three-dimensional location and light-curve evolution. We also apply our scoring algorithm to other transient candidates in the localization volume of S250818k and find that, at all times after the discovery of SN 2025ulz, there are ≥4 candidates with a score comparable to SN 2025ulz, indicating that the kilonova search may have benefited from the additional follow-up of other candidates. During future kilonova searches, this type of scoring algorithm will be useful to rule out contaminating transients in real time, optimizing the use of valuable telescope resources.

The Astrophysical Journal LettersVol. 994(2)
Northwestern University (US), University of North Carolina at Chapel Hill (US), University of Arizona (US), Northern Arizona University (US), Queen's University (CA), Cornell University (US), Purdue University West Lafayette (US), University of Utah (US), W.M. Keck Observatory (US), University of Florida (US), University of California San Diego (US), University of Warwick (GB), Gemini North Observatory (US), Max Planck Institute for Astronomy (DE), Center for Astrophysics Harvard & Smithsonian (US), Centro Brasileiro de Pesquisas Físicas (BR), The NSF AI Institute for Artificial Intelligence and Fundamental Interactions (US), Arizona State University (US), Monash University (AU), Massachusetts Institute of Technology (US), University of Maryland, College Park (US), Michigan State University (US), University of California, Berkeley (US)
Openalex Percentile: Top 7%
Gamma-ray bursts and supernovae
3.46
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