AT2025ulz and S250818k: Leveraging DESI spectroscopy in the hunt for a kilonova associated with a sub-solar mass gravitational wave candidate

TL;DR

This paper demonstrates how pre-existing DESI spectroscopy enables rapid, spectroscopically informed follow-up of a sub-threshold gravitational-wave candidate (S250818k) by characterizing AT2025ulz's host galaxy at $z=0.084840$. Using FastSpecFit and Prospector, the authors derive a dusty, star-forming host with $M_*/M_\odot\approx 10^{9.92}$, $\mathrm{SFR}\sim 0.5\,M_\odot\,\mathrm{yr}^{-1}$, and a small AGN contribution, situating the host within populations of CC SN and short GRB hosts. They compute an overlap integral $\log_{10}\mathcal{I} \approx 3.9-4.2$ to assess association with the GW event, and perform spectroscopic host subtraction to reveal SN-like features, illustrating DESI's value for rapid vetting and for guiding targeted follow-up with smaller telescopes. The study also catalogs DESI redshifts of other candidates, shows the utility of 3D localization in prioritizing hosts, and discusses the broader impact of DESI on multi-messenger astronomy, including strategies for future observing runs and follow-up coordination. Overall, the work highlights a practical, scalable path to improve counterpart identification and rejection in time-domain, GW-era astronomy by leveraging DESI's extensive spectroscopic dataset.

Abstract

On August 18th, 2025, the LIGO--Virgo--KAGRA collaboration reported a sub-threshold gravitational wave candidate detection consistent with a sub-solar-mass neutron star merger, denoted S250818k. An optical transient, AT2025ulz, was discovered within the localization region. AT2025ulz initially appeared to meet the expected behavior of kilonova (KN) emission, the telltale signature of a binary neutron star merger. The transient subsequently rebrightened after $\sim$\,$5$ days and developed spectral features characteristic of a Type IIb supernova. In this work, we analyze the observations of the host galaxy of AT2025ulz obtained by the Dark Energy Spectroscopic Instrument (DESI). From the DESI spectrum, we obtain a secure redshift of $z = 0.084840 \pm 0.000006$, which places the transient within $2σ$ of the gravitational wave distance and results in an integral overlap between the gravitational wave alert and the transient location of $\log_{10}\mathcal{I} \approx 3.9-4.2$. Our analysis of the host galaxy's spectral energy distribution reveals a star-forming, dusty galaxy with stellar mass ${\sim} 10^{10}~M_\odot$, broadly consistent with the population of both short gamma-ray bursts and core-collapse supernova host galaxies. We also present our follow-up of DESI-selected candidate host galaxies using the Fraunhofer Telescope at the Wendelstein Observatory, and show the promise of DESI for associating or rejecting candidate electromagnetic counterparts to gravitational wave alerts. These results emphasize the value of DESI's extensive spectroscopic dataset in rapidly characterizing host galaxies, enabling spectroscopic host subtraction, and guiding targeted follow-up.

Figures (7)

• Figure 1: False-color image in $grz$ bands of AT2025ulz from the first epoch of Wendelstein Observatory imaging on August 18, 2025 GCN41421Hall2025sn. The transient is visibly off-center from the galaxy. Star forming regions of the galaxy are noticeable as bluer patches. In this color image the transient is visually enhanced by co-adding with a difference image with respect to Legacy Survey data Dey2019AJ.
• Figure 2: Left: Spectral energy distribution fit from FastSpecFit, combining Legacy Survey and WISE photometry with DESI spectra. The lower panel shows the stellar population model spectrum, with key emission lines labeled. Right: Gaussian fits to individual spectral features used to measure line properties. The dark curve shows the best-fit model, while the faint lines represent the DESI data. We derive a redshift $z=0.084840 \pm 0.000006$.
• Figure 3: A BPT baldwin_classification_1981 analysis of the host of AT2025ulz (violet) based on its DESI spectrum and including for comparison the host galaxies of known gamma ray bursts (GRB) with an associated candidate KN (GRBs 160821B and 211211A) observed by DESI. The galaxy is a standard star forming galaxy, similar to the hosts of known GRB-KN. Notably, the host of AT2017gfo (associated to GW170817) has weak AGN lines that are not present in any of the other galaxies palmese_evidence_2017. The background contours are made from the FastSpecFit value added catalog for DESI DR2 data FastSpecFit (version 3.2.0; 2023ascl.soft08005Mdesi_collaboration_desi_2025Moustakas20XXfastspecfit). Separation lines are drawn from kewley_theoretical_2001, kauffmann_host_2003, and kewley_host_2006.
• Figure 4: The best-fit host galaxy SED derived using prospector. Johnson2019 Model photometry is shown as orange squares and compared to the observed photometry (black circles). The top panel shows the residuals compared to the best-fit SED (red line).
• Figure 5: Corner plot showing the posterior distribution of the host galaxy parameters derived using prospectorJohnson2019. The median point is placed in red.