In fact, it has been easier for astronomers to spot neutrinos from the far reaches of the Universe than those from the Milky Way. “We now have more information on the extragalactic sources than the Galactic ones,” says IceCube spokesperson Francis Halzen, a physicist at the University of Wisconsin–Madison. “Which is amazing.”
Mapping the Galaxy using high-energy neutrinos was always a top priority, says Halzen, in part because it could shed light on a long-standing mystery. Interstellar space is filled with high-energy cosmic rays, and researchers don’t know for sure where they come from. But one thing is clear: when they collide with other matter, such particles must produce an abundance of high-energy neutrinos — the ones that IceCube was designed to see, each packing energies of 500 gigaelectronvolts or more.
So the team had a massive sorting job to find the high-energy Galactic neutrinos. The researchers analysed 59,592 detections between May 2011 and May 2021, in the energy range of 500 GeV to several petaelectronvolts. They estimated that only 7% of those were neutrinos originating from deep space.
Although the bursts and other stellar explosions release stupendous amounts of energy in a short time, they might not accelerate individual protons to the near-light speed required to make cosmic rays and neutrinos, Halzen says.The long-term aftermath of a supernova could be a different matter, however. Over centuries, the expanding shock waves from such an explosion could act as pinball machines, accelerating protons to higher energies.