Ultrahigh energy cosmic rays are the highest-energy particles in the Universe, whose energies are more than a million times what can be achieved by humans.
Professor Farrar proposes that binary neutron star mergers are the source of all or most ultrahigh energy cosmic rays; this scenario can account for the heretofore inexplicable narrow rigidity range of ultrahigh energy cosmic rays because the jets of binary neutron star mergers are generated by a gravitationally driven dynamo and thus are nearly identical due to the narrow range of binary neutron star masses. Image credit: Osaka Metropolitan University / L-INSIGHT, Kyoto University / Ryuunosuke Takeshige.
While the existence of ultrahigh energy cosmic rays has been known for nearly six decades, astrophysicists have not succeeded in formulating a satisfactory explanation for their origin that explains all the observations — until now.
A new theory introduced by New York University’s Glennys Farrar provides a viable and testable explanation for how ultrahigh energy cosmic rays are created.
“After six decades of effort, the origin of the mysterious highest-energy particles in the Universe may finally have been identified,” Professor Farrar said.
“This insight gives a new tool for understanding the most cataclysmic events of the Universe: two neutron stars merging to form a black hole, which is the process responsible for the creation of many precious or exotic elements, including gold, platinum, uranium, iodine, and xenon.”
Professor Farrar proposes that ultrahigh energy cosmic rays are accelerated in the turbulent magnetic outflows of binary neutron star mergers — spewed out from the merger remnant, prior to formation of the final black hole.
The process simultaneously generates powerful gravitational waves — some already detected by scientists at the LIGO-Virgo Collaboration.
“This work explains, for the first time, two of the most mysterious features of ultrahigh energy cosmic rays: the tight correlation between their energy and electric charge, and the extraordinary energy of a handful of the very highest energy events,” Professor Farrar said.
“Stemming from the study are two consequences that can provide experimental validation in future work:
(i) the very highest energy cosmic rays originate as rare ‘r-process’ elements, such as xenon and tellurium, motivating a search for such a component in the ultrahigh energy cosmic ray data;
(ii) extremely high-energy neutrinos, originating from ultrahigh energy cosmic ray collisions, are necessarily accompanied by the gravitational wave produced in the parent neutron star merger.”
The study appears in the journal Physical Review Letters.
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Glennys R. Farrar. 2025. Binary Neutron Star Mergers as the Source of the Highest Energy Cosmic Rays. Phys. Rev. Lett 134, 081003; doi: 10.1103/PhysRevLett.134.081003
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