The origins of extremely high-energy particles that fill the Universe - such as protons, electrons, and neutrinos - remain one of the longest-standing mysteries in modern astrophysics. A leading hypothesis suggests that "explosive transients," including massive stellar explosions (supernovae) and tidal disruption events (TDE) caused by stars being torn apart by black holes, could be the cosmic engines driving these energetic particles. Yet, this idea has never been rigorously tested.

A research team has conducted the first systematic search for optical counterparts to a neutrino "multiplet," a rare event in which multiple high-energy neutrinos are detected from the same direction within a short period. The event was observed by the IceCube Neutrino Observatory, a massive detector buried deep within the Antarctic ice.

The team was led by Seiji Toshikage, a graduate student at Tohoku University's Graduate School of Science, Shigeo Kimura, a professor and TI-FRIS Fellow at Tohoku University's Frontier Research Institute for Interdisciplinary Sciences (FRIS), and Masaomi Tanaka, also from the Tohoku University's Graduate School of Science.

By analyzing wide-field optical data that coincided both spatially and temporally with the neutrino multiplet, the researchers sought visible evidence of possible astrophysical sources. However, their investigation found no supernovae, TDEs, or other explosive transients at the corresponding times and positions.

Figure: Arrival direction of a high-energy neutrino multiplet event determined by the IceCube experiment, overlaid on the visible night sky (generated with Stellarium). The right panel shows a zoomed-in optical image of the same region. The red ellipse indicates the 1σ uncertainty estimated by IceCube. ©Stellarium, Zwicky Transient Facility

The study was published in The Astrophysical Journal on October 23, 2025.

Publication Details:

Title: The First Search for Astronomical Transient as a Counterpart of a Month-timescale IceCube Neutrino Multiplet Event

Authors: Seiji Toshikage, Shigeo S. Kimura, Nobuhiro Shimizu, Masaomi Tanaka, Shigeru Yoshida, Wataru Iwakiri, Tomoki Morokuma

Journal: The Astrophysical Journal

DOI: 10.3847/1538-4357/adfedf

https://doi.org/10.3847/1538-4357/adfedf