⚛️ phenomenology

Nuclei in high-energy neutrino sources: A multimessenger study of in-source propagation

This paper utilizes Monte Carlo simulations of nuclear and electromagnetic cascades in the NGC 1068 source to demonstrate how joint gamma-ray and neutrino observations, including a re-analysis of archival COMPTEL data, can constrain the nuclear composition and physical conditions of high-energy astrophysical environments.

Original authors: AmirFarzan Esmaeili, Arman Esmaili, Pasquale Dario Serpico

Published 2026-06-24

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CC BY 4.0

Original authors: AmirFarzan Esmaeili, Arman Esmaili, Pasquale Dario Serpico

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, chaotic kitchen where high-energy particles are being cooked up. For a long time, scientists believed that the "ingredients" being thrown into the cosmic pot were almost exclusively protons (the simplest, lightest building blocks of matter). But this new paper asks a simple, yet profound question: What if the chef is actually tossing in heavy, complex ingredients like iron or zirconium nuclei instead?

The authors, Amir Farzan Esmaeili, Arman Esmaili, and Pasquale Dario Serpico, investigate this idea using NGC 1068, a distant galaxy with a supermassive black hole at its center, as their test kitchen. They want to see how the "recipe" changes if the starting ingredients are heavy nuclei rather than just simple protons.

Here is how their study works, explained through everyday analogies:

1. The "Hidden" Kitchen and the Smoke

Scientists have detected high-energy neutrinos (ghostly particles that pass through everything) coming from NGC 1068. Usually, when you cook something that creates neutrinos, you also expect to see gamma rays (high-energy light) coming out of the same pot.

However, when we look at NGC 1068 with our gamma-ray telescopes, we don't see the expected amount of light. It's like walking into a kitchen where you hear the sizzling and smell the food (neutrinos), but you can't see the smoke or steam (gamma rays).

The Old Theory: The kitchen is just very far away, and the smoke gets absorbed by the air before it reaches us.
The Paper's Insight: The kitchen itself is so crowded and dense that the smoke gets trapped and reprocessed inside the room before it can escape. The authors call this an "opaque" source.

2. The "Heavy Suit" vs. The "Light Runner"

The core of the paper is a simulation of what happens when these heavy nuclei enter this crowded kitchen.

The Proton (The Light Runner): Imagine a light runner sprinting through a crowded room. They might bump into a few people, but they keep moving fast and relatively unscathed.
The Heavy Nucleus (The Heavy Suit): Now imagine a person wearing a massive, heavy suit of armor (like Iron or Zirconium) trying to run through that same room.
- The Crash: As the heavy suit runs, it smashes into the crowd (photons) much harder. It gets knocked apart, shedding pieces of armor (breaking into smaller fragments) until it's just a pile of loose parts (free protons and neutrons).
- The Energy Cost: While the heavy suit is falling apart, it loses a lot of energy to the crowd. Some of this energy turns into "smoke" (electromagnetic radiation) that gets trapped in the room.

3. The Two Scenarios: Small Room vs. Big Hall

The authors tested two different sizes for this "kitchen" (the source region) to see how the heavy suit behaves:

Scenario A: The Tiny, Crowded Room (Compact Source)

If the room is very small and dense, the heavy suit gets smashed apart almost instantly.
The Result: Because the heavy suit breaks down so quickly, it releases a lot of neutrons (neutral particles). Neutrons are like ghosts; they don't get slowed down by the crowd's electric fields. They zip straight through to the exit and turn into neutrinos very efficiently.
The Payoff: In this tiny room, using heavy ingredients actually produces more neutrinos for the same amount of energy and creates less trapped smoke (gamma rays) than using simple protons. It's a more efficient way to make neutrinos.

Scenario B: The Large, Spacious Hall (Extended Source)

If the room is larger, the heavy suit doesn't get smashed apart immediately. It spends a long time running around in its heavy armor.
The Result: While wearing that heavy armor, it drags against the crowd, losing a huge amount of energy to friction (creating lots of trapped smoke/gamma rays). It takes much longer to break down into neutrons.
The Payoff: In this big hall, using heavy ingredients is actually worse. You need to put in way more energy to get the same amount of neutrinos, and you end up creating a massive amount of trapped smoke that we should be able to see.

4. The Detective Work: The "MeV" Clue

The paper makes a crucial point about how to solve this mystery. Since the "smoke" (gamma rays) is trapped and reprocessed, it doesn't come out as high-energy light (TeV) but as lower-energy light (MeV).

The authors went back to old data from a telescope called COMPTEL (which looked at the sky in the 1990s and 2000s) and re-analyzed it with modern tools. They found that the "smoke" levels in the MeV range are very low.

The Verdict: If the source were a large hall with heavy ingredients, the smoke would be too thick, and we would have seen it. Since we don't see it, the "heavy ingredient" theory is only possible if the source is a tiny, compact room. If the source is bigger, the heavy ingredients are ruled out because they would create too much visible smoke.

Summary

This paper is like a cosmic detective story. It suggests that if the universe is cooking up heavy nuclei (like iron) to make neutrinos, the kitchen must be very small and dense. If the kitchen were larger, the heavy ingredients would create a "smoke signal" (gamma rays) that our telescopes would have already spotted.

By re-examining old data and running detailed simulations, the authors show that the MeV gamma-ray band (a specific color of light we can't see with our eyes) is the key to figuring out what kind of "ingredients" these cosmic engines are actually using. They conclude that while heavy nuclei are a possibility, they are only allowed if the source is compact; otherwise, the universe is likely sticking to the simpler recipe of protons.

Technical Summary: Nuclei in High-Energy Neutrino Sources: A Multimessenger Study of In-Source Propagation

Problem Statement
The discovery of high-energy astrophysical neutrinos by IceCube has raised critical questions regarding the nature of their sources. While standard models assume these neutrinos originate from proton-proton ($pp$) or proton-photon ( $p\gamma$ ) interactions, the lack of corresponding TeV–GeV gamma-ray counterparts suggests these sources are "hidden" or opaque to high-energy photons. Previous studies have largely assumed a pure proton composition for accelerated particles. However, the chemical composition of the injected particles (e.g., light vs. heavy nuclei) remains an open question. If the source contains a significant fraction of nuclei, the in-source propagation involves complex nuclear cascades (photodisintegration) and electromagnetic processes that alter the resulting neutrino and gamma-ray yields. The central problem addressed is how relaxing the pure proton hypothesis and considering nuclear composition affects the multimessenger phenomenology, specifically the ratio of neutrino to gamma-ray fluxes and the required source luminosity.

Methodology
The authors perform detailed Monte Carlo simulations of nuclear and electromagnetic cascades within a high-energy astrophysical source, using the Seyfert galaxy NGC 1068 as a representative case study. The simulation is divided into two stages:

Photonuclear Interactions and Nuclear Cascade: The code tracks the evolution of primary nuclei (from $Z=1$ $Z = 1$ to $Z=90$ $Z = 90$ ) through a background photon field. It includes:
- Photodisintegration (PD): Modeled using TALYS/TENDL-2023 cross-sections for heavy nuclei and dedicated parametrizations for light nuclei ( $A \le 9$ ). This includes exclusive breakup channels (e.g., $(\gamma, n)$ , $(\gamma, p)$ , $(\gamma, \alpha)$ ) and nuclear resonance fluorescence.
- Photomeson (PM) Production: Modeled using the SOPHIA event generator with a superposition approximation for nuclei.
- Bethe-Heitler (BH) Pair Production: Treated as a continuous energy loss process for charged nuclei.
- Pair Production with Capture (PPC): A novel inclusion of pair production where the electron is captured into the L-shell (PPCL), followed by prompt atomic de-excitation emitting line photons.
- Nuclear Decay: Included for completeness but found to be negligible for the timescales considered.
Electromagnetic Cascade: Secondary photons, electrons, and positrons from stage (i) are propagated through the radiation field. This includes standard Inverse Compton Scattering (ICS) and Electron Pair Production (EPP), as well as rare processes like double pair production (DPP) and muon pair production (MPP), though the latter are noted as negligible in the specific energy ranges studied.

The source is modeled as a homogeneous sphere of radius $R$ embedded in a disk-corona photon field (UV disk + X-ray corona). The study varies the source size ( $R$ ) and the injected nuclear composition (protons, neutrons, and various nuclei like Carbon, Iron, and Zirconium) to map the optical depths and energy partitioning.

Key Contributions

Numerical Framework: The paper presents a new numerical treatment of nuclear-induced cascades that extends photonuclear processes up to $Z=90$ isotopes and explicitly accounts for pair production with capture in the L-shell, a process previously neglected in such contexts.
Systematic Analysis of Composition: It provides a comprehensive analysis of how the injected nuclear composition alters the neutrino-to-gamma-ray yield ratio, demonstrating that this relationship is non-monotonic and highly dependent on the source's optical depth (photon density and size).
COMPTEL Re-analysis: The authors perform a dedicated re-analysis of archival COMPTEL data for NGC 1068 using modern frameworks (GammaLib/ctools), deriving updated 95% confidence level upper limits in the MeV band that are approximately a factor of 2 tighter than previous bounds in the lower energy bins.

Results
The study identifies two distinct regimes based on the source compactness (photon density):

Compact/Dense Regime ( $R \lesssim 2 R_S$ ):
- The source is optically thick to photodisintegration. Heavy nuclei are rapidly fragmented into free nucleons (protons and neutrons) before escaping.
- A significant fraction of the energy is transferred to free neutrons. Since neutrons do not suffer Bethe-Heitler (BH) energy losses, the energy available for neutrino production is preserved.
- Outcome: Heavy nuclei injection is more efficient at producing neutrinos per unit of cosmic-ray luminosity than protons. Consequently, matching the observed IceCube neutrino flux requires a lower total cosmic-ray luminosity for heavy nuclei. The resulting electromagnetic cascade (MeV–GeV gamma rays) is weaker (by ~28–35%) compared to the proton case, thereby relaxing constraints from gamma-ray observations.
Extended/Rarefied Regime ( $R \gtrsim 5 R_S$ ):
- Photodisintegration is incomplete; a substantial fraction of the injected energy remains locked in charged nuclear fragments throughout the source crossing.
- These heavy fragments experience enhanced BH energy losses (scaling as $Z^2/A$ ) compared to protons at the same energy per nucleon.
- Outcome: Heavy nuclei injection is less efficient at producing neutrinos. To match the IceCube flux, a higher cosmic-ray luminosity (20–30% increase) is required. Furthermore, the enhanced BH losses and higher luminosity lead to a significantly stronger electromagnetic cascade (factor of ~2.0–2.5 higher gamma-ray flux), causing heavy-nuclei scenarios to overshoot existing MeV–GeV upper limits more severely than proton scenarios.

Significance and Claims
The paper claims that the nuclear composition of accelerated particles is a critical, yet often overlooked, parameter in modeling high-energy neutrino sources. The significance of the work lies in demonstrating that the interpretation of multimessenger data (neutrinos vs. gamma rays) is inextricably linked to the source's compactness and the injected composition.

Specifically, the authors argue that:

The MeV gamma-ray band is a powerful diagnostic tool for "hidden" neutrino sources because the electromagnetic cascade reprocesses high-energy photons into this band.
Current MeV data (specifically the re-analyzed COMPTEL limits) already provide mild but meaningful constraints on the source composition and size.
In compact sources, heavy nuclei can explain the neutrino flux with less total power and less gamma-ray emission, potentially resolving tension with gamma-ray non-detections. Conversely, in less compact sources, heavy nuclei are disfavored by gamma-ray limits.
Future MeV gamma-ray facilities (e.g., COSI, AMEGO-X, e-ASTROGAM) will be essential for sharpening these probes and distinguishing between proton and nuclear injection scenarios in the emerging era of multimessenger astrophysics.

The authors remain modest, noting that their model is a simplified "toy-model" of the disk-corona system and that future work will need to incorporate synchrotron losses and more complex geometries (2D/3D) to fully capture the source physics.

1. The "Hidden" Kitchen and the Smoke

2. The "Heavy Suit" vs. The "Light Runner"

3. The Two Scenarios: Small Room vs. Big Hall

4. The Detective Work: The "MeV" Clue

Summary

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