For decades, the origin of ultrahigh-energy cosmic rays has puzzled scientists. The mystery only deepened with the discovery of neutrinos and gamma rays of similar energies. Now, two researchers believe they have a solution that links all three of them in a single unified theory.
As reported in Nature Physics, the researchers propose that the cosmic rays are given such high energies by being accelerated by powerful black hole jets. As the cosmic rays move through clusters of galaxies, they collide with other particles, which generate neutrinos and gamma-rays as a consequence.
"The fact that the measured intensities of very high-energy neutrinos, ultrahigh-energy cosmic rays, and high-energy gamma rays are roughly comparable tempted us to wonder if these extremely energetic particles have some physical connections," co-author Professor Kohta Murase of Penn State University said in a statement. "The new model suggests that very high-energy neutrinos and high-energy gamma rays are naturally produced via particle collisions as daughter particles of cosmic rays, and thus can inherit the comparable energy budget of their parent particles. It demonstrates that the similar energetics of the three cosmic messengers may not be a mere coincidence."
The cosmic rays have energies higher than what we can produce on Earth, even with the Large Hadron Collider (the most powerful particle accelerator we have). The collider can produce energies in the order of trillions of electron volts, sending particles to incredibly high speeds at almost the speed of light. The particles produced by supermassive black holes, the so-called active galactic nuclei, can go even faster.
"Our work demonstrates that the ultrahigh-energy cosmic rays escaping from active galactic nuclei and their environments such as galaxy clusters and groups can explain the ultrahigh-energy cosmic-ray spectrum and composition. It also can account for some of the unexplained phenomena discovered by ground-based experiments," first author Ke Fang, from the University of Maryland, explained.
"Simultaneously, the very high-energy neutrino spectrum above one hundred million mega-electronvolts can be explained by particle collisions between cosmic rays and the gas in galaxy clusters and groups. Also, the associated gamma-ray emission coming from the galaxy clusters and intergalactic space matches the unexplained part of the diffuse high-energy gamma-ray background that is not associated with one particular type of active galactic nucleus."
The team look forward to their model being tested by future observatories. By better locating the origin of these signals, they hope to confirm (or not) the finding of their model.