ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Subthreshold Subsolar Gravitational-wave Trigger

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Kasliwal, MM orcid iconORCID: 0000-0002-5619-4938, Ahumada, T orcid iconORCID: 0000-0002-2184-6430, Stein, R orcid iconORCID: 0000-0003-2434-0387, Karambelkar, V orcid iconORCID: 0000-0003-2758-159X, Hall, XJ orcid iconORCID: 0000-0002-9364-5419, Singh, A orcid iconORCID: 0000-0003-2091-622X, Fremling, C orcid iconORCID: 0000-0002-4223-103X, Metzger, BD orcid iconORCID: 0000-0002-4670-7509, Bulla, M orcid iconORCID: 0000-0002-8255-5127, Swain, V orcid iconORCID: 0000-0002-7942-8477, Antier, S orcid iconORCID: 0000-0002-7686-3334, Pillas, M orcid iconORCID: 0000-0003-3224-2146, Busmann, M orcid iconORCID: 0009-0001-0574-2332, Freeburn, J orcid iconORCID: 0009-0006-7990-0547, Karpov, S orcid iconORCID: 0000-0003-0035-651X, Bochenek, A orcid iconORCID: 0009-0008-2714-2507, O’Connor, B orcid iconORCID: 0000-0002-9700-0036, Perley, DA orcid iconORCID: 0000-0001-8472-1996, Akl, D orcid iconORCID: 0009-0006-4358-9929, Anand, S orcid iconORCID: 0000-0003-3768-7515 et al (2025) ZTF25abjmnps (AT2025ulz) and S250818k: A Candidate Superkilonova from a Subthreshold Subsolar Gravitational-wave Trigger. Astrophysical Journal Letters, 995 (2). ISSN 2041-8205

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Abstract

On 2025 August 18, the LIGO-Virgo-KAGRA collaboration reported gravitational waves from a subthreshold 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, ZTF 25abjmnps (AT2025ulz), which 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 ZTF 25abjmnps and S250818k. Theoretical models have been proposed wherein subsolar 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 multiwavelength dataset in the context of the superkilonova picture. Future higher-significance gravitational-wave detections of subsolar neutron star mergers with extensive electromagnetic follow-up would conclusively resolve this tantalizing multimessenger association.

Item Type: Article
Uncontrolled Keywords: 5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5109 Space sciences
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: American Astronomical Society
Date of acceptance: 3 November 2025
Date of first compliant Open Access: 23 April 2026
Date Deposited: 23 Apr 2026 14:42
Last Modified: 23 Apr 2026 14:42
DOI or ID number: 10.3847/2041-8213/ae2000
URI: https://researchonline.ljmu.ac.uk/id/eprint/28445
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