Meet “Amaterasu”: Astronomers detect highest energy cosmic ray since 1991

Astronomers concerned with the Telescope Array experiment in Utah’s West Desert have detected an ultra-high-energy cosmic ray (UHECR) with a whpping power stage of 244 EeV, based on a new paper revealed within the journal Science. It is essentially the most energetic cosmic ray detected since 1991, when astronomers detected the so-called “Oh-My-God’ particle, with energies of an much more spectacular 320 EeV. Astronomers have dubbed this newest occasion the “Amaterasu” particle, after the Shinto sun goddess stated to have created Japan. One may even name it the “Oh-My-Goddess” particle.

Cosmic rays are extremely energetic subatomic particles touring via area close to the velocity of sunshine. Technically, a cosmic ray is simply an atomic nucleus made up of a proton or a cluster of protons and neutrons. Most originate from the Solar, however others come from objects exterior our photo voltaic system. When these rays strike the Earth’s environment, they break aside into showers of different particles (each positively and negatively charged).

They had been first discovered in 1912 by Austrian physicist Victor Hess through a sequence of ascents in a hydrogen balloon to take measurements of radiation within the environment with an electroscope. He discovered that the speed of ionization was a very good thrice the speed at sea stage, thereby disproving a competing concept that this radiation got here from the rocks of  Earth. When you’ve ever seen a cloud chamber in a science museum, cosmic ray tracks appear to be wispy little white strains, just like tiny jet contrails.

Cosmic rays are available a broad vary of energies, with the least energetic being the most typical. These had been the cosmic rays Hess detected, and are those most definitely to point out up in a museum cloud chamber. There’s a theoretical restrict, proposed in 1965, to only how energetic a cosmic ray ought to be: not more than 50 EeV coming from greater than 300 million gentle years from Earth. That is due to the cosmic microwave background radiation, the afterglow of the Huge Bang that pervades the universe, found in 1964. Any cosmic rays touring additional than that may be destroyed through interactions with the CMB earlier than they reached Earth’s detectors. It is referred to as the GZK cutoff after the scientists who proposed it (Kenneth Greisin, Georgiy Zatsepin, and Vadim Kuzmin).

The 1991 discovery of the “Oh-My-God” particle challenged that prevailing concept, hitting the Earth’s environment at very close to the velocity of sunshine and apparently touring from the route of the Perseus constellation within the northern hemisphere. It carried the power equal of a bowling ball dropped from shoulder peak, packed tightly right into a subatomic particle. Astronomers have not seen its equal since, though they’ve detected dozens of occasions that qualify as UHECRs over the following many years.

However what may very well be the supply of such UHECRs, able to accelerating the subatomic particles to such spectacular speeds? Even a supernova would not be capable of do that. One doable supply is an increasing shock wave from a cosmic-scale explosion—say, a black gap ripping aside a star and producing an enormous jet of plasma—by which particles traverse magnetic fields time and again and choose up power as they journey via area. One other candidate is energetic galactic nuclei (AGNs) sometimes discovered on the middle of galaxies and assumed to include a supermassive black gap. AGNs produce highly effective jets of superheated plasma accompanied by shock waves.

Different strategies embrace gamma ray bursts (themselves arising from an unknown supply) or intense areas of star formation referred to as starburst galaxies. It does not assist that the trajectories of UHECRs are bent by magnetic fields en path to our detectors on Earth, making it troublesome to reconstruct the route they traveled and thereby pinpoint an origin level within the sky. Astronomers thought they’d recognized a few intriguing sizzling spots again in 2017, one in Centaurus A and the opposite in a galaxy known as M82 within the Ursa Main constellation. However confidence within the former hotspot has weakened since 2019 because the variety of UHCERs detected from there seems to be dropping.

The Telescope Array consists of over 500 floor detectors organized in a sq. grid that covers some 270 sq. miles (700 sq. kilometers) simply exterior of Delta, Utah. It has picked up greater than 30 UHECRs because it started operation. Even so, co-author Toshihiro Fujii of Osaka Metropolitan College in Japan “thought there must have been a mistake” when the experiment picked up the “Amaterasu”  particle on Might 27, 2021. Extraordinary claims require extraordinary proof, because the mantra goes, so the detection and trajectory evaluation weren’t introduced till a convention final fall, with the paper solely now simply coming in Science.

Like its 1991 predecessor, astronomers are baffled as to the place the particle got here from. Tracing its trajectory led them to an empty space of area referred to as the “Native Void” bordering our Milky Approach galaxy.  “The particles are so excessive power, they shouldn’t be affected by galactic and extra-galactic magnetic fields. It’s best to be capable of level to the place they arrive from within the sky,” said co-author John Matthews, Telescope Array co-spokesperson on the College of Utah. “However within the case of the Oh-My-God particle and this new particle, you hint its trajectory to its supply and there’s nothing excessive power sufficient to have produced it. That’s the thriller of this—what the heck is occurring?”

We’d study extra as soon as astronomers end increasing the Telescope Array, including 500 new scintillator detectors which might broaden the detection space to 1,100 sq. miles (2,900 sq. kilometers). That ought to enhance how usually they detect such UHECRs.

“These occasions appear to be they’re coming from utterly completely different locations within the sky. It’s not like there’s one mysterious supply,” said co-author John Belz, additionally with the College of Utah. “It may very well be defects within the construction of spacetime, colliding cosmic strings. I imply, I’m simply spit-balling loopy concepts that individuals are developing with as a result of there’s not a traditional clarification. Perhaps magnetic fields are stronger than we thought, however that disagrees with different observations that present they’re not sturdy sufficient to supply important curvature at these ten-to-the-twentieth electron volt energies. It’s an actual thriller.”

DOI: Science, 2023. 10.1126/science.abo5095  (About DOIs).

Itemizing picture by Osaka Metropolitan College/L-INSIGHT, Kyoto College/Ryuunosuke Takeshige