ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Sub-threshold Sub-Solar Gravitational Wave Trigger
Abstract
On August 18, 2025, the LIGO-Virgo-KAGRA collaboration reported gravitational waves from a sub-threshold binary neutron star merger. If astrophysical, this event would have a surprisingly low chirp mass, suggesting that at least one neutron star was below a solar mass. The Zwicky Transient Facility mapped the coarse localization and discovered a transient, ZTF25abjmnps (AT2025ulz), that was spatially and temporally coincident with the gravitational wave trigger. The first week of follow-up suggested properties reminiscent of a GW170817-like kilonova. Subsequent follow-up suggests properties most similar to a young, stripped-envelope, Type IIb supernova. Although we cannot statistically rule out chance coincidence, we undertake due diligence analysis to explore the possible association between ZTF25abjmnps and S250818k. Theoretical models have been proposed wherein sub-solar neutron star(s) may form (and subsequently merge) via accretion disk fragmentation or core fission inside a core-collapse supernova i.e. a ``superkilonova". Here, we qualitatively discuss our multi-wavelength dataset in the context of the superkilonova picture. Future higher significance gravitational wave detections of sub-solar neutron star mergers with extensive electromagnetic follow-up would conclusively resolve this tantalizing multi-messenger association.
Keywords
Cite
@article{arxiv.2510.23732,
title = {ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Sub-threshold Sub-Solar Gravitational Wave Trigger},
author = {Mansi M. Kasliwal and Tomas Ahumada and Robert Stein and Viraj Karambelkar and Xander J. Hall and Avinash Singh and Christoffer Fremling and Brian D. Metzger and Mattia Bulla and Vishwajeet Swain and Sarah Antier and Marion Pillas and Malte Busmann and James Freeburn and Sergey Karpov and Aleksandra Bochenek and Brendan O'Connor and Daniel A. Perley and Dalya Akl and Shreya Anand and Andrew Toivonen and Sam Rose and Theophile Jegou du Laz and Chang Liu and Kaustav Das and Sushant Sharma Chaudhary and Tyler Barna and Aditya Pawan Saikia and Igor Andreoni and Eric C. Bellm and Varun Bhalerao and S. Bradley Cenko and Michael W. Coughlin and Daniel Gruen and Daniel Kasen and Adam A. Miller and Samaya Nissanke and Antonella Palmese and Jesper Sollerman and Niharika Sravan and G. C. Anupama and Smaranika Banerjee and Sudhanshu Barway and Joshua S. Bloom and Tomas Cabrera and Tracy Chen and Chris Copperwheat and Alessandra Corsi and Richard Dekany and Nicholas Earley and Matthew Graham and Patrice Hello and George Helou and Lei Hu and Yves Kini and Ashish Mahabal and Frank Masci and Tanishk Mohan and Natalya Pletskova and Josiah Purdum and Yu-Jing Qin and Nabeel Rehemtulla and Anirudh Salgundi and Yuankun Wang},
journal= {arXiv preprint arXiv:2510.23732},
year = {2025}
}
Comments
Accepted by ApJL on 11/4/25