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

TL;DR

This work tests whether a candidate after a gravitational-wave event truly represents a kilonova or a more common transient, using SN 2025ulz as a case tied to the sub-threshold GW event S250818k. It combines a comprehensive multiwavelength follow-up with a novel, quantitative scoring algorithm that fuses 2D localization, 3D distance, host/asteroid checks, and photometric evolution to assess KN likelihood in real time. The analysis finds SN 2025ulz to be a Type IIb supernova, not a kilonova, and demonstrates that early KN-like photometric behavior can be misleading without full light-curve evolution and spectroscopic confirmation. The authors demonstrate the scoring framework’s real-time utility, quantify the candidate landscape within the S250818k localization, and announce TROVE, a public web tool to standardize and accelerate follow-up decisions in future poorly localized events.

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.

Figures (12)

• Figure 1: The IGWN (International Gravitational Wave Network) skymap credibility contours (orange: 50%, white: 90%) of S250818k, and the locations of all 121 candidates we found within the 95% credibility region from the Transient Name Server (TNS). The color of the point represents the overall score from our vetting, which is described in \ref{['sec:search']}.
• Figure 2: A false color image of the HST WFC3 observations of SN 2025ulz. We include the F606W image from 26 Aug (blue; $\delta t \sim 8$ days), the F110W image from 27 Aug (green; $\delta t \sim 9$ days), and the F160W image from 28 Aug 2025 (red; $\delta t \sim 10$ days). We highlight the position of SN 2025ulz showing its close in projection to the core of its host galaxy.
• Figure 3: Top: A comparison of the $gri$ light curves for SN 2025ulz (black), SN 2016gkg tartaglia_progenitor_2017, SN 2022hnt farah_when_2025, and AT 2017gfo villar_combined_2017. For SN 2025ulz, We choose an explosion date of MJD=60904 based on our shock cooling modeling (see \ref{['sec:early-lc-model']}). The best fit (i.e., lowest sum of the residual squared) $r$-band KN model kasen_origin_2017 to SN 2025ulz for the early time ($\delta t \leq 6$ days) light curve decline is shown as a solid line. We also show the $r$- (black dashed line) and $g$-band (black dotted line) models for SN 2016gkg from piro_numerically_2017. At phase $\delta t \lesssim 6$ days all four observed light curves appear to redden as they fade rapidly at around the same rate. At $\delta t \gtrsim 6$ days, the light curves of SN 2025ulz and the other SNe IIb begin to re-brighten while the KN light curve continues to fade rapidly. Bottom: The $g-r$ color evolution of SN 2025ulz as compared to AT 2017gfo, SN 2016gkg, SN 2022hnt, and 7 other SNe IIb (SN 1993J, richmond_ubvri_1994; SN 2008ax, Pastorello2008; SN 2011dh, Arcavi2011dh; SN 2011ei, Dan2013; SN 2011fu, Morales2015_2011fu; SN 2013df, morales2014_2013df; SN 2024uwq, Subrayan_2024uwq). The color evolution of SN 2025ulz is very similar to the rest of the SNe IIb population.
• Figure 4: Comparison of the MMT optical spectrum of SN 2025ulz (black) to the type IIb SN 2022hnt farah_when_2025, SN 2016gkg tartaglia_progenitor_2017 and the kilonova AT 2017gfo pian_spectroscopic_2017, all at similar phases. The low opacity colored regions behind the spectra show the 1-sigma uncertainty and the grey vertical strips are regions of telluric features. The spectral features from SN 2025ulz appear most similar to SN 2022hnt and do not show any similarity with AT 2017gfo. Most notably, the broad P-Cygni H$\alpha$ feature highlighted in dark blue, present in both SN 2025ulz and SN 2022hnt, is not expected, nor seen, in the KN spectrum.
• Figure 5: Fits of the $griI$ and HST F336W (which is comparable to Swift/UVOT $U$-band) light curves of SN 2025ulz with a shock cooling modeling SW2017, assuming two polytropic indices: $n=3/2$ (left) and $n=3$ (right). The best-fit explosion date, with uncertainties, and the detection date of S250818k are shown as grey and purple dashed lines, respectively.