_version_ 1866909873097146368
author Hall, Xander J.
Palmese, Antonella
O'Connor, Brendan
Gruen, Daniel
Busmann, Malte
Gassert, Julius
Hu, Lei
Hernandez, Ignacio Magana
Aguilar, Jessica Nicole
Amsellem, Ariel
Ahlen, Steven
Banovetz, John
BenZvi, Segev
Bianchi, Davide
Brooks, David
Castander, Francisco Javier
Claybaugh, Todd
Cuceu, Andrei
Dey, Arjun
Doel, Peter
Faba-Moreno, Jennifer
Ferraro, Simone
Font-Ribera, Andreu
Forero-Romero, Jaime E.
Gutierrez, Gaston
Guillou, Laurent Le
Joyce, Dick
Kisner, Theodore
Kremin, Anthony
Lahav, Ofer
Lamman, Claire
Landriau, Martin
Levi, Michael
de la Macorra, Axel
Manera, Marc
Meisner, Aaron
Miquel, Ramon
Moustakas, John
Nadathur, Seshadri
Prada, Francisco
Perez-Rafols, Ignasi
Rossi, Graziano
Sanchez, Eusebio
Schlegel, David
Schubnell, Michael
Sprayberry, David
Tarle, Gregory
Weaver, Benjamin Alan
Zhou, Rongpu
Zou, Hu
author_facet Hall, Xander J.
Palmese, Antonella
O'Connor, Brendan
Gruen, Daniel
Busmann, Malte
Gassert, Julius
Hu, Lei
Hernandez, Ignacio Magana
Aguilar, Jessica Nicole
Amsellem, Ariel
Ahlen, Steven
Banovetz, John
BenZvi, Segev
Bianchi, Davide
Brooks, David
Castander, Francisco Javier
Claybaugh, Todd
Cuceu, Andrei
Dey, Arjun
Doel, Peter
Faba-Moreno, Jennifer
Ferraro, Simone
Font-Ribera, Andreu
Forero-Romero, Jaime E.
Gutierrez, Gaston
Guillou, Laurent Le
Joyce, Dick
Kisner, Theodore
Kremin, Anthony
Lahav, Ofer
Lamman, Claire
Landriau, Martin
Levi, Michael
de la Macorra, Axel
Manera, Marc
Meisner, Aaron
Miquel, Ramon
Moustakas, John
Nadathur, Seshadri
Prada, Francisco
Perez-Rafols, Ignasi
Rossi, Graziano
Sanchez, Eusebio
Schlegel, David
Schubnell, Michael
Sprayberry, David
Tarle, Gregory
Weaver, Benjamin Alan
Zhou, Rongpu
Zou, Hu
contents 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.
format Preprint
id arxiv_https___arxiv_org_abs_2510_23723
institution arXiv
publishDate 2025
record_format arxiv
spellingShingle AT2025ulz and S250818k: Leveraging DESI spectroscopy in the hunt for a kilonova associated with a sub-solar mass gravitational wave candidate
Hall, Xander J.
Palmese, Antonella
O'Connor, Brendan
Gruen, Daniel
Busmann, Malte
Gassert, Julius
Hu, Lei
Hernandez, Ignacio Magana
Aguilar, Jessica Nicole
Amsellem, Ariel
Ahlen, Steven
Banovetz, John
BenZvi, Segev
Bianchi, Davide
Brooks, David
Castander, Francisco Javier
Claybaugh, Todd
Cuceu, Andrei
Dey, Arjun
Doel, Peter
Faba-Moreno, Jennifer
Ferraro, Simone
Font-Ribera, Andreu
Forero-Romero, Jaime E.
Gutierrez, Gaston
Guillou, Laurent Le
Joyce, Dick
Kisner, Theodore
Kremin, Anthony
Lahav, Ofer
Lamman, Claire
Landriau, Martin
Levi, Michael
de la Macorra, Axel
Manera, Marc
Meisner, Aaron
Miquel, Ramon
Moustakas, John
Nadathur, Seshadri
Prada, Francisco
Perez-Rafols, Ignasi
Rossi, Graziano
Sanchez, Eusebio
Schlegel, David
Schubnell, Michael
Sprayberry, David
Tarle, Gregory
Weaver, Benjamin Alan
Zhou, Rongpu
Zou, Hu
High Energy Astrophysical Phenomena
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.
title AT2025ulz and S250818k: Leveraging DESI spectroscopy in the hunt for a kilonova associated with a sub-solar mass gravitational wave candidate
topic High Energy Astrophysical Phenomena
url https://arxiv.org/abs/2510.23723