University of Copenhagen researchers map Milky Way neutrino flow to Earth

Researchers at the University of Copenhagen have created the most detailed map yet of neutrino flow from the Milky Way, offering a new lens to study high-energy particles that originate in the galaxy’s stars. The study, published in *Physical Review D*, combines stellar evolution models with data from the European Space Agency’s Gaia telescope to predict where neutrinos — often called "ghost particles" — are produced and how they reach Earth. The work is a notable step in understanding the invisible signals that could reveal secrets about stellar interiors and cosmic physics.

University of Copenhagen's neutrino map

The research team, led by Pablo Martínez-Miravé and Irene Tamborra at the Niels Bohr Institute, developed a model that links neutrino production to the distribution of stars in the Milky Way. By analyzing the energy and mass of stars, they determined that the galactic center, particularly regions a few thousand light-years from Earth, emits the strongest neutrino signals. "Most neutrinos reaching Earth come from the region around the galactic center, especially from areas a few thousand light-years from Earth," Martínez-Miravé said. "This means the best chance of detecting neutrino signals is when looking towards the galactic center."

Neutrinos, which rarely interact with matter, offer a unique window into stellar processes. Unlike light or other radiation, they escape directly from a star’s core, carrying information about its internal conditions. "Neutrinos carry information straight from the interior of stars," Tamborra said. "If we learn to harness them, they can give us new insights into stellar life cycles and the structure of our galaxy in a way no other source can."

The model also highlights how neutrino flux varies with star type. Younger, more massive stars contribute disproportionately to the neutrino signal, while thermal processes and nuclear reactions within stars shape their energy distribution. This roadmap could guide future neutrino detectors, helping them target the strongest signals and improve sensitivity to low-energy neutrinos.

Microquasars and the cosmic ray 'knee'

While the Copenhagen study focuses on neutrinos, other research points to microquasars, binary systems with black holes and companion stars, as key sources of high-energy cosmic rays. The Large High Altitude Air Shower Observatory (LHAASO) in China identified five microquasars emitting ultra-high-energy gamma rays and protons, some exceeding 10 PeV. These findings resolve a decades-old mystery about the "knee" in the cosmic ray energy spectrum, a sharp drop in particle counts above 3 PeV. "Microquasars are capable of accelerating particles to PeV levels," said researchers from the Chinese Academy of Sciences. "This explains the 'knee' and provides evidence for black hole jet systems as major cosmic ray accelerators."

One microquasar, SS 433, was found to produce protons with energies exceeding 1 PeV, releasing energy comparable to four trillion hydrogen bombs per second. Another, V4641 Sgr, generated gamma rays reaching 0.8 PeV. These observations challenge previous assumptions that supernova remnants alone could account for such extreme energies. "Supernova remnants cannot explain the energies seen at and above the 'knee,'" said LHAASO researchers. "Microquasars, however, can."

Future directions and collaborative efforts

The University of Copenhagen’s neutrino map and LHAASO’s microquasar findings highlight the growing interplay between particle physics and astrophysics. Neutrino detectors, such as the LZ experiment, are poised to test predictions from these models, while observatories like Fermi and Chandra continue to refine cosmic ray source catalogs. "Any small mismatch between prediction and measurement can become a clue," Tamborra said. "These deviations could point toward new particle behavior or unknown physical laws."

Collaborative efforts across institutions, ranging from the Niels Bohr Institute to the Chinese Academy of Sciences, underscore the complexity of unraveling cosmic mysteries. As detectors grow more sensitive and models more precise, the Milky Way’s "ghost particles" may soon reveal long-hidden truths about the universe’s most energetic processes.