While the Universe may appear serene and inspiring at first glance, it is actually filled with particles traveling at nearly the speed of light that possess immense energy. These consist primarily of atomic nuclei and subatomic particles, such as protons, electrons, and neutrinos, which constantly bombard Earth. The origin of these particles remains one of the longest-standing mysteries in modern astrophysics. A leading theory is that they are created by extreme events, such as supernovae and tidal disruption events (TDEs), which occur when stars are ripped apart by black holes.
In this scenario, explosive transient events and powerful gravitational forces accelerate particles to relativistic speeds. However, this theory has never been rigorously tested. In a recent study, a research team led by Tohoku University conducted the first systematic search for cosmic counterparts for a "neutrino multiplets" event detected by the IceCube Neutrino Observatory in Antarctica. The results of their study are helping to narrow the search for the origins of the most energetic particles in the Universe.
The team was led by Seiji Toshikage, a graduate student at Tohoku University's Astronomical Institute and the Graduate School of Science. He was joined by Shigeo Kimura, a professor at Tohoku University's Frontier Research Institute for Interdisciplinary Sciences (FRIS), Masaomi Tanaka of the Tohoku University's Graduate School of Science, and researchers from the International Center for Hadron Astrophysics (ICHA) and the Astronomy Research Center at the Chiba Institute of Technology. Their research was presented in a study that appeared on Oct. 23rd in The Astrophysical Journal.
A multiplet is a rare event where multiple high-energy neutrinos are detected from the same direction within a short period. In this case, the detections took place over the course of a month. The team began their search for possible sources using data obtained by the Zwicky Transient Facility (ZTF), which uses an extremely wide-field of view camera to detect transient optical events in the night sky. They then looked for visible evidence of astrophysical events that coincided with the neutrino "multiplet event" detected by the IceCube Neutrino Observatory.
While their investigation found no trace of supernovae, TDEs, or other explosive transient events coinciding with the neutrino detection, this null detection was still highly informative. Essentially, these results allowed the team to place the tighter constraints to date on explosive events that could produce neutrino multiplets - specifically, the magnitude of their brightness and the duration of the events. Moreover, they are bringing the astronomical community one step closer to resolving one of the most fundamental mysteries in astrophysics. As Toshikage said in a Tohoku University press release:
Although we didn't find any transient sources this time, our results show that even non-detections can provide powerful insights. They help us refine our models and guide future searches for the true sources of high-energy neutrinos.
For their next step, the team plans to conduct rapid follow-up observations to identify additional optical counterparts to neutrino multiplets detected by the IceCube collaboration. Building on the analysis methods developed for this study, they hope their efforts will identify the cosmic sources of all the high-energy particles that permeate the Universe.
Further Reading: Tohoku University, The Astrophysical Journal