Physicists detect second-most powerful cosmic ray from beyond our galaxy

New Delhi: Researchers have detected the second-most powerful cosmic ray to have come from outside our galaxy. But the exact origin of this "ultra-high-energy" particle from outer space remains a mystery.

Observed in Utah, this is believed to be the most powerful cosmic ray detection since the 'Oh-My-God' particle in 1991 that left physicists puzzled.

Named 'Amaterasu' particle after the sun goddess in Japanese mythology, the second-most powerful cosmic ray has energy equivalent to "dropping a brick on one's toe from waist's height", the international team of physicists mentioned in their study published in the journal Science.

The energy of the particle was measured at 2.4 x 10^20 electron Volts (eV) and was detected on May 27, 2021, the researchers said.

Cosmic rays are charged particles like protons or electrons travelling through the universe at nearly the speed of light. The charged particles could be remnants of celestial events that have broken down matter to its subatomic constituents.

Because the particles are charged, their flight path resembles a ball in a pinball machine as they zigzag against the electromagnetic fields through the cosmic microwave background, the researchers said.

Referring to 'Amaterasu', John Matthews, study co-author said, "The particles are so high energy, they shouldn't be affected by galactic and extra-galactic magnetic fields. You should be able to point to where they come from in the sky."

This research was conducted at the Telescope Array at the University of Utah (US), which was also witness to the discovery of the highest-energy cosmic ray ever in 1991 during the 'Fly's Eye' experiment. Dubbed as the 'Oh-My-God' particle, the most powerful cosmic ray shocked astrophysicists as it was believed that nothing in our galaxy had the power to produce it and it was theoretically not possible for cosmic rays travelling to Earth from other galaxies.

The scientists said they are yet to find a "conventional explanation" for these events and believe that this rare phenomena might pertain to particle physics unknown to science.

"But in the case of the Oh-My-God particle and this new particle, you trace its trajectory to its source and there's nothing high energy enough to have produced it. That's the mystery of this..." said Matthews.

Particles with 'Oh-My-God' and 'Amaterasu' energy blast through intergalactic space were relatively unbent. Only the most powerful of celestial events can produce them, the team said.

"Things that people think of as energetic, like supernova, are nowhere near energetic enough for this. You need huge amounts of energy, really high magnetic fields to confine the particle while it gets accelerated," said Matthews.

Analysing the cosmic ray composition for clues of its origins, the researchers said the 'Amaterasu' particle is likely a proton, owing to its relatively unbent trajectory.

Heavier particles like iron nuclei have more charge and are more susceptible to bending in a magnetic field than a lighter particle such as a proton from a hydrogen atom, they said.

Further, the physicists said that according to particle physics, a cosmic ray with energy beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff, such as 'Amaterasu', is too powerful for the microwave background to distort its path. GZK cutoff is the maximum energy a proton can hold travelling over long distances before the effect of interactions of the microwave background radiation take their energy.

However, the mystery of the 'Amaterasu' particle deepened when the scientists found that back tracing its trajectory pointed to empty space.

"Maybe magnetic fields are stronger than we thought, but that disagrees with other observations that show they're not strong enough to produce significant curvature at these ten-to-the-twentieth electron volt energies," said study co-author John Belz, a professor at the University of Utah. "It's a real mystery."