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