Ultraheavy Ultrahigh-Energy Cosmic Rays

This paper proposes that ultraheavy nuclei could constitute the highest-energy cosmic rays, offering a consistent explanation for the Amaterasu particle, constraints on source energy generation rates, and the spectral tension between Telescope Array and Pierre Auger Observatory data, while predicting distinct shower maximum depths for future experimental verification.

The Gist

Imagine the universe is filled with a constant, invisible rain of particles called cosmic rays. Most of these are like gentle drizzles, but occasionally, a single drop hits us with the energy of a baseball thrown by a professional pitcher, yet it's the size of a single atom. These are Ultra-High-Energy Cosmic Rays (UHECRs). For over 50 years, scientists have been trying to figure out where these "superballs" come from and what they are made of.

This paper proposes a new idea: some of the most energetic particles we've ever seen might not be made of common elements like hydrogen or iron, but of "Ultra-Heavy" (UH) nuclei. Think of these as cosmic "gold bars" or "lead bricks" compared to the usual "feathers" (lighter particles) we expect.

Here is the story of the paper, broken down into simple concepts:

1. The Problem: The "Heavy" Mystery

Scientists have two big telescopes watching the sky: the Pierre Auger Observatory (in Argentina) and the Telescope Array (in Utah). They see the same "rain" of cosmic rays, but they disagree on exactly how many high-energy drops there are and what they are made of.

Recently, the Telescope Array spotted a particle so energetic it was named the "Amaterasu" particle (after a Japanese sun goddess). It was a record-breaker. The question is: What is this thing made of?

2. The New Idea: The "Heavyweight" Travelers

Usually, scientists think these high-energy particles are protons (hydrogen nuclei) or maybe iron. But this paper suggests that some of them could be Ultra-Heavy nuclei—atoms heavier than iron, like Platinum or Selenium.

The Analogy: The Marathon Runners
Imagine a marathon where runners have to pass through a field of "energy sponges" (background radiation in space).

• Light runners (Protons): They get tired very quickly. They lose their speed (energy) fast and can't run very far.
• Medium runners (Iron): They last a bit longer but still get worn down.
• Heavy runners (Ultra-Heavy Nuclei): Because they are so massive and dense, they are surprisingly tough. They can run much farther without losing their speed.

The paper calculates that these "heavy runners" can travel distances that lighter particles simply cannot. This means they could come from sources that are further away or rarer, and they could still arrive at Earth with record-breaking energy.

3. The "Amaterasu" Particle

The authors suggest that the "Amaterasu" particle might be one of these heavy runners.

• If it were a proton: It would have to come from a very specific, nearby location to survive the trip.
• If it is a heavy nucleus: It could have come from a different direction, perhaps from a violent explosion in a nearby galaxy, because its "heavy armor" protected it during the journey.

4. Where do they come from?

The paper looks at the "factories" that might make these heavy particles. They suggest two main cosmic events:

• Collapsars: Massive stars that collapse into black holes (often creating Gamma-Ray Bursts).
• Neutron Star Mergers: Two incredibly dense stars crashing into each other.

These events are like cosmic forges that can smash atoms together to create heavy elements (like gold or platinum) and then blast them out into space at incredible speeds. The paper finds that the energy these events produce is just enough to explain the number of these heavy cosmic rays we see.

5. Solving the Disagreement

The two telescopes (Auger and Telescope Array) have been arguing about the data. The paper suggests that if we add these "heavy runners" into the mix, and assume one of them came from a nearby explosion (like a low-luminosity Gamma-Ray Burst just 5 million light-years away), it helps explain why the Telescope Array sees more high-energy particles than Auger does. It's like realizing one observer is standing closer to a firework display than the other.

6. How do we know?

The paper doesn't just guess; they ran complex computer simulations. They created a new "rulebook" for how these heavy atoms interact with space (since standard software didn't handle atoms heavier than iron well). They simulated the journey of these particles and compared the results to real data.

The Prediction:
If these heavy particles are real, they should change how the "showers" of particles look when they hit Earth's atmosphere. Specifically, the "depth" of the shower (how deep it goes before peaking) should be different for heavy nuclei than for iron.

• The Test: Future telescopes (like AugerPrime and the Global Cosmic Ray Observatory) will be able to measure this depth. If the showers look "shallower" (or behave differently) at the highest energies, it will confirm that these heavy particles are indeed the ones arriving.

Summary

This paper argues that the most energetic particles in the universe might be made of super-heavy atoms (heavier than iron). These heavy atoms are tough enough to travel long distances through space without losing their energy. This idea helps explain a mysterious record-breaking particle ("Amaterasu") and might finally settle the debate between two major cosmic ray observatories. The next step is to wait for new data to see if the "heavy runners" are actually winning the race.

Technical Summary

Technical Summary: Ultraheavy Ultrahigh-Energy Cosmic Rays

Problem Statement
The origin of ultrahigh-energy cosmic rays (UHECRs) remains an unresolved mystery, particularly regarding their composition and the sources capable of accelerating particles to energies exceeding $10^{20}$ eV. While the observed UHECR spectrum exhibits a hardening at the "ankle" ($\sim 4$ EeV) and a cutoff near $50$ EeV, discrepancies persist between the energy spectra measured by the Pierre Auger Observatory (Auger) and the Telescope Array (TA). Notably, TA has reported an excess at the highest energies, including the detection of the "Amaterasu" particle ($244 \pm 29$ EeV), which lies in a region where standard proton and intermediate-mass nuclei models struggle to explain the flux without violating energy loss constraints. Furthermore, the composition of UHECRs is debated; while Auger data suggests a mixed composition becoming heavier at high energies, the specific role of ultraheavy (UH) nuclei—defined here as nuclei heavier than the iron group ($A > 56$)—has not been systematically constrained in the context of intergalactic propagation.

Methodology
The authors investigate the propagation of UH nuclei (specifically Selenium, Tellurium, and Platinum, representing r-process peaks) as UHECRs.

1. Propagation Modeling: The study utilizes the CRPropa 3.2 code to simulate the intergalactic propagation of UHECRs. Since CRPropa 3.2 lacks modules for nuclei with mass numbers $A > 56$, the authors generated new photodisintegration cross-section tables using the nuclear reaction network Talys 1.96. They extended the code to include 2434 isotopes with $Z \le 92$ and $A \le 238$, incorporating decay data from NuDat 3.
2. Energy Loss Mechanisms: The study calculates total energy loss lengths considering photodisintegration (dominant at very high energies), Bethe-Heitler pair production (dominant in the $10^{20}$ eV range for UH nuclei), photomeson production, and adiabatic losses.
3. Spectral Fitting: The authors perform a combined fit to the energy spectra and composition data (specifically the depth of shower maximum, $X_{max}$, and its fluctuations) from Auger and TA. They test two composition models:
   • Model A: Conventional nuclei (p, He, O, Si) + UH nuclei (Se, Te, Pt).
   • Model B: Conventional nuclei (p, He, O, Si, Fe) + UH nuclei (Se, Te, Pt).
     The injection spectrum is assumed to be a power law with an exponential cutoff in rigidity ($R = E/Z$).
4. Constraints: Using the chi-square method and the Akaike Information Criterion (AIC), the authors derive constraints on the energy generation rate density ($Q_{UH-UHECR}$) required to fit the data. They also backtrack the arrival direction of the Amaterasu particle under different nuclear species assumptions using a Galactic magnetic field model.

Key Results

• Propagation Lengths: UH nuclei possess significantly longer energy loss lengths at energies $\lesssim 300$ EeV compared to protons and intermediate-mass nuclei. While Bethe-Heitler pair production is the dominant energy loss mechanism for UH nuclei in the $1-5 \times 10^{20}$ eV range, photodisintegration becomes dominant at higher energies. Crucially, UH nuclei can travel distances exceeding the GZK horizon for protons and the energy loss length of iron-group nuclei, allowing them to reach Earth from more distant or rarer sources.
• Composition and Spectra:
  • Auger Data: The inclusion of UH nuclei does not significantly improve the fit to Auger data compared to models with only conventional nuclei. The derived upper limits on the energy generation rate density for UH-UHECRs are $Q_{Auger} \lesssim (0.1 - 15) \times 10^{42}$ erg Mpc$^{-3}$ yr$^{-1}$.
  • TA Data: Including UH-UHECRs as a second population provides a better fit to the TA energy spectrum and composition data (rescaled AIC $\Delta_i \lesssim 2$). The best-fit energy generation rate densities are $Q_{TA} \lesssim (1.4 - 5.6) \times 10^{43}$ erg Mpc$^{-3}$ yr$^{-1}$, approximately three times higher than the Auger-derived limits.
• The Amaterasu Particle: The authors demonstrate that the Amaterasu particle can be explained as a UH-UHECR event. Unlike protons or iron nuclei, which would require a source in the local void (inconsistent with the particle's arrival direction), a UH nucleus (e.g., Platinum) could originate from outside the local void or near the supergalactic plane due to its larger atomic number and reduced magnetic deflection.
• Source Candidates: The derived energy generation rate densities are consistent with the energy budgets of collapsars (including long GRBs and hypernovae) and compact binary mergers (neutron star-neutron star and neutron star-black hole). The required efficiency ($\epsilon_{CR} \sim 0.1\% - 10\%$) is physically plausible for these transient sources.

Significance and Claims
The paper claims to provide the first detailed study on the propagation of UH-UHECRs and the first constraints on their contribution to the highest-energy cosmic rays.

• Alleviating Tension: The study suggests that the spectral tension between Auger and TA can be alleviated by considering an enhanced contribution from a nearby transient source (e.g., a low-luminosity GRB or BNS merger at $\sim 5$ Mpc) that injects UH nuclei.
• Testable Predictions: The model predicts that the mean depth of shower maximum ($\langle X_{max} \rangle$) for UH-UHECRs will be lower than that of iron nuclei at energies beyond 100 EeV. This signature can be tested with future composition measurements from AugerPrime and the Global Cosmic Ray Observatory (GCOS).
• Source Nature: The existence of UH nuclei at the highest energies would indicate that UHECRs are produced by transient sources (like collapsars and mergers) rather than steady sources like active galactic nuclei.
• Multimessenger Potential: The scenario implies specific multimessenger signatures, including neutrinos from nuclear beta decay and gamma rays from nuclear de-excitation and Bethe-Heitler cascades, which could be targets for future detectors like the Cherenkov Telescope Array.

The authors maintain a modest stance, noting that while UH-UHECRs offer a viable explanation for the TA excess and the Amaterasu event, current data primarily serves to constrain their contribution rather than definitively prove their dominance. Future composition measurements at the highest energies are cited as the critical next step for validating these models.