The Type IIn SN 2025cbj coincidence with the high-energy neutrino IceCube-250421A

¹ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
² Institute for Advanced Study in Physics, Zhejiang University, Hangzhou 310027, China
³ Institute for Astronomy, School of Physics, Zhejiang University, Hangzhou 310027, China
⁴ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁵ Institute for Applications of Machine Learning and Intelligent Systems (IAMLIS), Munich University of Applied Sciences, Lothstr. 34, D-80335 Munich, Germany
⁶ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Str. 1, D-85748 Garching, Germany

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Received: 2 December 2025
Accepted: 12 February 2026

Abstract

Context. The origins of astrophysical high-energy neutrino flux remain uncertain. Core-collapse supernovae (CCSNe) with strong circumstellar material (CSM) interactions (i.e., Type IIn, a.k.a. SNe IIn) are compelling candidates for explaining efficient hadronic acceleration and neutrino production.

Aims. We investigate the possible association between the SNe IIn SN2025cbj and the IceCube high-energy neutrino IceCube-250421A. We assess whether the observed properties of the SN would enable an appreciable neutrino yield.

Methods. We combined rapid optical follow-ups with LAST and archival ZTF photometry with spectroscopy from LT/SPRAT and MMT/BINOSPEC to characterize the SN’s evolution and CSM interaction. We estimated the explosion and peak times from an early light-curve fitting and quantified the chance-coincidence probability with resampling simulations that scramble neutrino right ascensions, while preserving declinations and error contours. Using a simple post-shock-breakout interaction model in a dense wind, we estimated the expected muon-neutrino yield for IceCube’s real-time Bronze stream.

Results. The spectra of SN2025cbj obtained after the neutrino epoch exhibit persistent narrow Balmer lines superposed on broad Lorentzian electron-scattering wings, consistent with a sustained dense CSM interaction. For the multimessenger association, resampling the simulations against the TNS catalog gives a chance-coincidence p-value of p ≃ 0.24 for observing k ≥ 1 events (and p ≃ 0.078 against the ZTF-BTS catalog). These values are sensitive to the size of the SN and neutrino samples and do not indicate a statistically significant multimessenger association. A post-breakout interaction scenario predicts an expected N_νμ ∼ 10⁻³ events in the IceCube Bronze alert stream over 96 days per this one candidate. We discuss the implications of these numbers and the possible biases that could affect these results.