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