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

TL;DR

This study examines a potential multi-messenger link between the Type IIn SN2025cbj and the IceCube-250421A neutrino, leveraging rapid optical follow-up, archival photometry, and spectroscopy to characterize the SN and assess neutrino production. By constraining explosion and peak times and applying resampling against SN catalogs, the authors quantify the chance coincidence probabilities and contextualize the SN within a post-breakout CSM interaction framework. They find the observed SN-neutrino coincidence to be consistent with random chance, yet they present a plausible, low-rate neutrino yield (~$N_{\nu_{\mu}} \sim 10^{-3}$ over ~76 days) under a dense wind scenario, highlighting biases and dependencies on sample size. The work underscores the value of large-scale, real-time, multi-wavelength follow-ups and future, larger neutrino detectors to robustly identify a population of neutrino-emitting SNe IIn.

Abstract

Context. The origin of the astrophysical high-energy neutrino flux remains uncertain. Core-collapse supernovae with strong CSM interaction (Type IIn) are compelling candidates for efficient hadronic acceleration and neutrino production. Aims. We investigate the possible association between the Type IIn supernova SN2025cbj and the IceCube high-energy neutrino IceCube-250421A, and assess whether the observed properties of the SN permit an appreciable neutrino yield. Methods. We combined rapid optical follow-up with LAST and archival ZTF photometry with spectroscopy from LT/SPRAT and MMT/BINOSPEC to characterize the SN evolution and CSM interaction. We estimated the explosion and peak times from 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 real-time Bronze stream. Results. Spectra of SN2025cbj obtained after the neutrino epoch show persistent narrow Balmer lines superposed on broad Lorentzian electron-scattering wings, consistent with sustained dense-CSM interaction. For the multi-messenger association, resampling simulations against the TNS catalog give a chance-coincidence probability for observing $k \ge 1$ events of $p \simeq 0.24$ (and $p \simeq 0.078$ against the ZTF-BTS catalog). These values are sensitive to the size of the SNe and neutrino samples. A post-breakout interaction scenario predicts an expected $N_{ν_μ} \sim 10^{-3}$ events in the IceCube Bronze alert stream over 76 days per this one candidate. We discuss the implications of these numbers and possible biases that may affect these results.

Figures (3)

• Figure 1: Upper: photometric light curves from LAST (blue), ZTF-$g$ (green), ZTF-$r$ (red), DDOTI/OAN (orange) with temporal x-axis centered on the fitted peak time at MJD 60754.5. The magenta marker is the Swift-UVOT measurement in the UVW1 band. The dashed purple vertical line marks the arrival of IceCube-250421A . Lower: Pseudo-bolometric light curve in the (4700-6231 $\AA$) range. The dot-dashed line shows the power-law fit of the early light curve decay after the peak ($\propto$ t$^{\alpha}$, $\alpha$ = -0.43).
• Figure 2: Liverpool telescope spectra of SN2025cbj. a narrow H$\alpha$ component can be identified in both epochs, below a strong galactic H $\alpha$ narrow feature. However, the lower resolution does not allow accurate velocity measurements or a positive identification of the H$\beta$ line. We note that the very narrow components (e.g., on top of the real narrow H$\alpha$ feature in the green spectrum) are noise residue, as well as the features in the blue end of both spectra.
• Figure 3: MMT spectrum of SN2025cbj. Zero velocity H$\alpha$ and H$\beta$ are marked with red and blue striped vertical lines, respectively. Zoom-ins of the H$\alpha$ and $\beta$ profiles with fits to a Gaussian+Lorentzian shape are added, showing a good fit aside from the blue-wing asymmetry in H$\alpha$. The best-fit velocities (i.e. line FWHM) for both the Gaussian ($v_{G}$) and Lorentzian ($v_{L}$) components are presented next to the fits.