An ultraheavy particle might assist clarify one of the crucial puzzling mysteries in fashionable astrophysics: the origin of probably the most energetic particles ever detected.
Ultrahigh-energy cosmic rays are particles from area that slam into Earth with energies far past something produced by human-built particle accelerators. Among the many most extraordinary examples is the “Amaterasu particle,” which was detected by the Telescope Array in Utah in 2021 and named for the solar goddess in Japanese mythology. Its reported vitality ranks it among the many strongest cosmic-ray occasions ever noticed, putting it in the identical uncommon class because the “Oh-My-God particle” recorded in 1991. But scientists nonetheless have no idea the place it got here from, and even precisely what it was.
Ultraheavy Cosmic Rays
New analysis led by scientists at Penn State and printed in Bodily Assessment Letters means that a few of the highest-energy cosmic rays could also be atomic nuclei heavier than iron. Atomic nuclei are the compact facilities of atoms, made up of protons and neutrons. They maintain nearly all of an atom’s mass whereas taking over solely a tiny a part of its whole quantity.
Based on the group’s calculations, these ultraheavy nuclei might lose vitality extra slowly than protons or lighter nuclei whereas crossing intergalactic area. Meaning they may survive the journey to Earth whereas nonetheless carrying excessive quantities of vitality. The work, carried out with collaborators on the Yukawa Institute for Theoretical Physics in Japan, Virginia Tech and different establishments, might assist scientists determine the varieties of cosmic objects highly effective sufficient to launch such particles.
“Ultrahigh-energy cosmic rays can solely be accelerated by a few of the strongest sources within the universe,” mentioned Kohta Murase, professor of physics and of astronomy and astrophysics within the Penn State Eberly Faculty of Science and the chief of the analysis group. “After we detect particular person cosmic-ray particles such because the Amaterasu particle right here on Earth, we are able to usually use their energies, arrival instructions and anticipated magnetic deflections to deduce their potential cosmic sources.”
The Amaterasu Particle Thriller
The Amaterasu particle has been particularly troublesome to clarify as a result of its estimated arrival route traces again to a cosmic void, a area of area with no clear supply able to producing ultrahigh-energy cosmic rays.
“The origins and acceleration mechanisms of ultrahigh-energy cosmic rays have been among the many greatest mysteries within the subject for greater than 60 years, for the reason that first instance was reported,” Murase mentioned.
These uncommon particles can exceed 100 exa-electron volts, or 100 quintillion electron volts. That makes them about seven orders of magnitude, or 10 million occasions, extra energetic than particles accelerated contained in the Giant Hadron Collider, the world’s largest and strongest particle accelerator. The Amaterasu particle was reported at about 240 exa-electron volts, giving one tiny cosmic-ray particle roughly the kinetic vitality of a fast-moving tennis ball. That makes it one of the crucial energetic cosmic rays ever detected.
“These highest-energy cosmic rays are thought to return from excessive astrophysical sources, like two neutron stars colliding or a large star collapsing,” Murase mentioned. “For a lot of cosmic-ray occasions taken collectively, their vitality distribution, arrival-direction sample and statistically inferred composition present necessary clues about the place these particles come from and the way they’re accelerated.”
Simulating Excessive Particles
To research what sorts of particles might nonetheless attain Earth at such extraordinary energies, the researchers ran detailed laptop simulations. They modeled how particles of various sizes would achieve or lose vitality whereas touring by intergalactic area.
“Our analysis confirmed that at energies similar to that of the Amaterasu particle, ultraheavy nuclei lose vitality extra slowly than protons or intermediate-mass nuclei, making them higher capable of survive cosmic distances and attain Earth at excessive energies,” Murase mentioned. “We aren’t saying that each one ultrahigh-energy cosmic rays are ultraheavy nuclei. But when a few of the highest-energy occasions are ultraheavy nuclei, that may affect how we seek for their sources.”
The group’s calculations additionally set new limits on how a lot these ultraheavy nuclei might contribute to the total inhabitants of noticed ultrahigh-energy cosmic rays.
Violent Cosmic Origins
“Probably the most promising websites for producing and accelerating such ultraheavy nuclei are large star deaths involving explosive collapse into black holes or strongly magnetized neutron stars, in addition to binary neutron-star mergers identified to be highly effective gravitational-wave emitters,” Murase mentioned. “These violent cosmic phenomena can even energy gamma-ray bursts which can be among the many most energetic explosions within the universe. A contribution from these sources might additionally assist clarify a potential distinction seen between the northern and southern skies within the ultrahigh-energy cosmic-ray spectrum. If ultraheavy nuclei contribute considerably on the highest energies, future information ought to point out a composition heavier than iron.”
Future observatories might be able to check these concepts. Murase mentioned next-generation services, together with the proposed AugerPrime in Argentina and the proposed World Cosmic Ray Observatory, might search for the expected signatures. Further theoretical work on cosmic explosions involving black holes and strongly magnetized neutron stars can also assist reveal the place ultrahigh-energy cosmic rays are born.
Together with Murase, the analysis group included B. Theodore Zhang, a postdoctoral researcher at Kyoto College’s Yukawa Institute for Theoretical Physics on the time of the analysis and a former Penn State postdoctoral researcher; Mukul Bhattacharya, an Eberly Postdoctoral Fellow at Penn State on the time of the analysis; and Nick Ekanger and Shunsaku Horiuchi, who had been at Virginia Tech on the time of the analysis.