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arXiv · 2510.17104

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

Noah Franz·Bhagya Subrayan·Charles D. Kilpatrick·Griffin Hosseinzadeh·David J. Sand·Kate D. Alexander·Wen-fai Fong·Collin T. Christy·Jeniveve Pearson·Tanmoy Laskar·Brian Hsu·Jillian Rastinejad·Michael J. Lundquist·Edo Berger·K. Azalee Bostroem·Clecio R. Bom·Phelipe Darc·Mark Gurwell·Shelbi Hostler Schimpf·Garrett K. Keating·Phillip Noel·Conor Ransome·Ramprasad Rao·Luidhy Santana-Silva·A. Souza Santos·Manisha Shrestha·Ramya Anche·Jennifer E. Andrews·Sanchayeeta Borthakur·Nathaniel R. Butler·Deanne L. Coppejans·Philip N Daly·Kathryne J. Daniel·Paul C. Duffell·Tarraneh Eftekhari·Carl E. Fields·Alexander T. Gagliano·Walter W. Golay·Aldana Grichener·Erika T. Hamden·Daichi Hiramatsu·Harsh Kumar·Vikram Manikantan·Raffaella Margutti·Vasileios Paschalidis·Kerry Paterson·Daniel E. Reichart·Mathieu Renzo·Kali Salmas·Genevieve Schroeder·Nathan Smith·Kristine Spekkens·Jay Strader·David E. Trilling·Nicholas Vieira·Benjamin Weiner·Peter K. G. Williams
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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 multi-wavelength 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 multi-wavelength observations of SN 2025ulz reveal that it is a type IIb supernova. 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 3D 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 $\geq 4$ 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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Noah Franz, Bhagya Subrayan, Charles D. Kilpatrick, Griffin Hosseinzadeh, David J. Sand, Kate D. Alexander, Wen-fai Fong, Collin T. Christy, Jeniveve Pearson, Tanmoy Laskar, Brian Hsu, Jillian Rastinejad, Michael J. Lundquist, Edo Berger, K. Azalee Bostroem, Clecio R. Bom, Phelipe Darc, Mark Gurwell, Shelbi Hostler Schimpf, Garrett K. Keating, Phillip Noel, Conor Ransome, Ramprasad Rao, Luidhy Santana-Silva, A. Souza Santos, Manisha Shrestha, Ramya Anche, Jennifer E. Andrews, Sanchayeeta Borthakur, Nathaniel R. Butler, Deanne L. Coppejans, Philip N Daly, Kathryne J. Daniel, Paul C. Duffell, Tarraneh Eftekhari, Carl E. Fields, Alexander T. Gagliano, Walter W. Golay, Aldana Grichener, Erika T. Hamden, Daichi Hiramatsu, Harsh Kumar, Vikram Manikantan, Raffaella Margutti, Vasileios Paschalidis, Kerry Paterson, Daniel E. Reichart, Mathieu Renzo, Kali Salmas, Genevieve Schroeder, Nathan Smith, Kristine Spekkens, Jay Strader, David E. Trilling, Nicholas Vieira, Benjamin Weiner, Peter K. G. Williams. 2025-10-26. Optimizing Kilonova Searches: A Case Study of the Type IIb SN 2025ulz in the Localization Volume of the Low-Significance Gravitational Wave Event S250818k. https://arxiv.org/abs/2510.17104

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