Washington: American and Japanese physicists have discovered evidence of "natural nuclear accelerators" at work in our Milky Way galaxy, based on an analysis of data from the world’s largest cosmic ray detector.
Cosmic rays of the highest energies were believed by physicists to come from remote galaxies containing enormous black holes capable of consuming stars and accelerating protons at energies comparable to that of a bullet shot from a rifle.
These protons — referred to individually as "cosmic rays" — travel through space and eventually enter our galaxy.
But earlier this year, physicists using the Pierre Auger Observatory in Argentina, the world’s largest cosmic ray observatory, published a surprising discovery.
They found that many of the energetic cosmic rays found in the Milky Way are not actually protons but nuclei — and the higher the energy, the greater the nuclei-to-proton ratio.
"This finding was totally unexpected because the nuclei, more fragile than protons, tend to disintegrate into protons on their long journey through space. Moreover, it is very unlikely that a cosmic accelerator of any kind would accelerate nuclei better than protons at these high energies," said Alexander Kusenko, UCLA professor of physics and astronomy and co-author of the Physical Review Letters research.
The resolution to the paradox of the nuclei’s origin comes from an analysis by Kusenko and colleagues.
They found that stellar explosions in our own galaxy can accelerate both protons and nuclei.
But while the protons promptly leave the galaxy, the heavier and less mobile nuclei become trapped in the turbulent magnetic field and linger longer.
"As a result, the local density of nuclei is increased, and they bombard Earth in greater numbers, as seen by the Pierre Auger Observatory," said Kusenko.
These ultra–high-energy nuclei have been trapped in the web of galactic magnetic fields for millions of years, and their arrival directions as they enter the Earth’s atmosphere have been "completely randomized by numerous twists and turns in the tangled field," he said.
"When the data came out, they were so unexpected that many people started questioning the applicability of known laws of physics at high energy," said Kusenko.
"The common lore has been that all ultra–high-energy cosmic rays must come from outside the galaxy. The lack of plausible sources and the arrival-direction anisotropy (the nuclei have different physical properties when measured in different directions) have been used as arguments in favor of extragalactic sources.
"However, since the cosmic rays in question turned out to be nuclei, the galactic field can randomize their arrival directions, taking care of the anisotropy puzzle. As for the plausible sources, the enormous stellar explosions responsible for gamma ray bursts can accelerate nuclei to high energies. When we put these two together, we knew we were on the right track. Then we calculated the spectra and the asymmetries, and both agreed with the data very well,” he added.
Kusenko hopes this research will enhance the understanding of "astrophysical archeology."
The research is published in the journal Physical Review Letters.