Identification of a Radio Counterpart to SN 2025ulz in the S250818k Localization Area

O’Dwyer, T; Corsi, A; Yadav, D; Mooley, KP; Baer-Way, R; Chandra, P; Hallinan, G; Kasliwal, MM; Rhodes, L; Smirnov, OM; Lazzati, D; van Leeuwen, J; Deller, A; Atri, P; Khanam, T

Journal article

Identification of a Radio Counterpart to SN 2025ulz in the S250818k Localization Area

Abstract:: Abstract On 2025 August 18, the LIGO – Virgo – KAGRA collaboration reported S250818k, a subthreshold gravitational-wave (GW) candidate consistent with a binary neutron star (NS) merger potentially involving a subsolar-mass NS. Optical follow-up by the Zwicky Transient Facility identified a transient temporally coinciding with the GW trigger that initially resembled a kilonova but was later classified as a young stripped-envelope Type IIb supernova (SN), dubbed SN 2025ulz. A key question is whether SN 2025ulz harbors fast, possibly collimated, nonthermal ejecta indicative of a central engine, as invoked in “superkilonova” scenarios linking subsolar-mass NSs to accretion-disk fragmentation or core fission. We present early-to-late-time multiband radio observations of SN 2025ulz obtained with the Karl G. Jansky Very Large Array as part of the JAGWAR program, complemented by observations with the upgraded Giant Metrewave Radio Telescope and MeerKAT. We detect a faint but significant radio counterpart to SN 2025ulz at 6−10 GHz. The data are consistent with nonthermal emission from SN ejecta interacting with circumstellar material. Our data are also consistent with emission from an off-axis jet peaking at ∼50−100 days after the GW trigger. While we cannot distinguish between these two interpretations, our radio detection motivates future systematic multiwavelength follow-up of core-collapse events coinciding with subsolar NS GW candidates, should the association between S250818k and SN 2025ulz be supported by offline GW analyses.

Publication status:: Published

Peer review status:: Peer reviewed

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APA Style

O’Dwyer, T., Corsi, A., Yadav, D., Mooley, K., Baer-Way, R., Chandra, P., Hallinan, G., Kasliwal, M., Rhodes, L., Smirnov, O., Lazzati, D., van Leeuwen, J., Deller, A., Atri, P., & Khanam, T. (2026). Identification of a Radio Counterpart to SN 2025ulz in the S250818k Localization Area. The Astrophysical Journal Letters, 1009(2), L44–L44.

MLA Style

O’Dwyer, T., et al. “Identification of a Radio Counterpart to SN 2025ulz in the S250818k Localization Area.” The Astrophysical Journal Letters, vol. 1009, no. 2, 2026, pp. L44–44.

Chicago Style

O’Dwyer, T., A. Corsi, D. Yadav, et al. 2026. “Identification of a Radio Counterpart to SN 2025ulz in the S250818k Localization Area.” The Astrophysical Journal Letters 1009 (2): L44–44.
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