For more than a century, scientists have known that Earth is constantly bombarded by cosmic rays, charged particles with staggering energies, far beyond what our accelerators can produce.
Yet their origins have remained hidden because cosmic rays are bent and scattered by magnetic fields during their journey across space.
Neutrinos, however, offer a way around this barrier. These ghostly particles rarely interact with matter and fly in straight lines from their sources. If we can trace where they come from, we can finally discover the source of cosmic rays. At the South Pole, the IceCube Neutrino Observatory, an immense detector buried deep under the ice, has been built to do just that.
Neutrinos are “ideal candidates for searching for the sources of cosmic radiation because they travel a more or less direct path from their source to Earth. There, they can be detected by IceCube,” the Ruhr University Bochum (RUB) team notes.
Researchers at RUB have created a new algorithm using which IceCube can now pinpoint neutrinos with far greater speed and accuracy than before.
A trick to improve neutrino path reconstruction
IceCube works by watching for the rare moment when a neutrino collides with an atom in Antarctic ice. Such a collision produces a flash of blue light. By comparing the timing and brightness of these flashes, scientists can reconstruct the neutrino’s path and work out where in the sky it came from.
Until now, however, these reconstructions were relatively rough, leaving astronomers with wide patches of sky to search, too large to reliably catch passing cosmic flares.
The RUB researcher changed that by overhauling IceCube’s analysis pipeline. Their system now issues an initial reconstruction in about 30 seconds, giving a quick estimate of the neutrino’s direction and energy that can be sent immediately to telescopes worldwide.
“We need 30 seconds to calculate the energy and direction of a neutrino, and immediately disseminate the information worldwide,” Anna Franckowiak, one of the researchers at RUB, said.
Then a slower, more detailed calculation follows, refining the trajectory with much greater precision. Depending on how much energy the neutrino leaves behind in the ice, the system switches between two mathematical approaches.
For lower-energy events, one method called SplineMPE provides sharp sky locations, while for higher-energy tracks, another method, Millipede Wilks, handles the complicated and irregular energy losses better. This hybrid strategy means astronomers get the best possible reconstruction for each event.
As a result, the regions of sky where a neutrino is thought to have come from are now about five times smaller for the 50 percent confidence area (the patch of sky from where neutrinos likely come), and about four times smaller for the 90 percent area, compared with the older system.
Checking previously suggested sources
The team didn’t stop at new detections. They re-analyzed more than a decade of IceCube’s archived alerts using the same improved methods, creating a cleaner and more reliable catalog.
This process showed how important precision is. For instance, some earlier associations, such as hints that neutrinos were linked to tidal disruption events (TDE), where black holes rip apart passing stars, disappeared once the paths were recalculated.
“After we improved our algorithm for trajectory reconstruction, we analyzed the events again, and the neutrino paths don’t match the positions where the tidal disruption events occurred,” Franckowiak said.
At the same time, the reanalysis uncovered a striking new clue. Two neutrinos, each carrying about 100 trillion electron volts of energy, were both consistent with having come from the same source, NGC 7469, a galaxy with an active core about 220 million light-years away.
“We estimate the possible neutrino flux from NGC 7469 under different assumptions. The result leaves open the possibility that either one or both of the neutrinos originated from the source,” the researchers note in their study.
The coincidence is intriguing, however, not conclusive. Other analysis suggest the strength of the signal depends on which reconstruction values are used, so for now the finding remains a promising but unproven lead.
Being able to trace neutrinos with this level of speed and accuracy is a turning point for high-energy astronomy. For the first time, astronomers can respond almost immediately when a neutrino is detected, pointing telescopes toward the right spot in the sky before a flare fades away.
If repeat detections are confirmed from the same object, whether it be an active galaxy like NGC 7469, a star-forming region, or another exotic source, it would finally reveal the long-sought birthplaces of cosmic rays.
Such a discovery would not only solve a century-old mystery but also expand our understanding of black holes and exploding stars.
This research is covered in three papers. You can read them here, here, and here.