⚛️ high-energy theory

PeV neutrons as origin of separated SS433 TeV signals

This paper proposes that the distant, separated TeV gamma-ray signals observed from the SS433 binary system originate from a century-old eruption of PeV relativistic neutron beams, whose beta decay produced high-energy electrons that subsequently generated gamma rays via Inverse Compton Scattering.

Original authors: D. Fargion, P. G. De Sanctis Lucentini, S. Turriziani, M. Y. Khlopov, D. Sopin

Published 2026-06-19

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CC BY 4.0

Original authors: D. Fargion, P. G. De Sanctis Lucentini, S. Turriziani, M. Y. Khlopov, D. Sopin

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

The Mystery: A Ghostly Twin Jet

Imagine a cosmic dance partner system called SS433. It consists of a massive black hole and a giant star orbiting each other. The black hole is "eating" the star, pulling gas away to form a swirling disk. This process shoots out two powerful beams of energy (jets) that spin like a lighthouse, creating a spiral trail of light. We have seen these jets in radio waves and X-rays for decades.

However, a few years ago, giant telescopes (like H.E.S.S., HAWC, and LHAASO) spotted something strange. They saw two new, bright beams of high-energy gamma rays appearing not near the black hole, but 75 to 150 light-years away from it.

It's as if you saw a lighthouse beam, and then suddenly, 100 miles away in the middle of a dark forest, a second, identical beam of light appeared out of nowhere, with no visible source connecting them. This is the "separated twin signal" that puzzles scientists.

The Old Theory vs. The New Idea

The Old Idea: Some scientists thought a massive shockwave traveled out from the black hole and re-accelerated particles to create this distant light. But the authors of this paper think this is too complicated and doesn't explain how the beam stays so straight (collimated) over such a long distance.

The New Idea (The "PeV Neutron" Model):
The authors, led by D. Fargion, propose a different story involving a "cosmic messenger" that is invisible until it gets old.

The Explosion (The Trigger): About 100 years ago (around the time of the World Wars), SS433 had a massive, rare explosion—a "nova-like flare."
The Transformation: During this explosion, the black hole shot out a beam of super-fast protons. As these protons zoomed through a hot bath of light near the black hole, they collided with photons and turned into neutrons.
- Analogy: Imagine a stream of charged bullets (protons) hitting a wall of mirrors (light). Instead of bouncing back, they transform into "ghost bullets" (neutrons) that lose their electric charge.
The Invisible Journey: Because neutrons have no electric charge, they don't get pushed around by magnetic fields in space. They travel in a perfectly straight line, invisible to our telescopes, for decades.
- Analogy: Think of these neutrons as a "stealth missile" flying through a storm. While charged particles (like protons) would get blown off course by the wind (magnetic fields), the neutron missile flies straight and true.
The "Beta Decay" Surprise: Neutrons are unstable. After traveling for about 100 years (covering 75–150 light-years), they begin to fall apart (decay).
- When a neutron decays, it splits into a proton, a neutrino, and a super-fast electron.
The Light Show: This newly created electron is moving at nearly the speed of light. As it zooms through space, it smashes into invisible infrared light particles, boosting them into high-energy TeV gamma rays.
- Analogy: It's like the "ghost missile" finally hitting a target and exploding into a shower of fireworks. The fireworks (gamma rays) are what our telescopes see.

Why This Fits the Evidence

The paper argues that this model explains three things that other models struggle with:

The Distance: The math shows that a neutron traveling at the right speed would take exactly 100 years to travel 75–150 light-years before decaying. This matches the timing of the explosion and the location of the new beams.
The Straight Line: Because the neutron is neutral, it doesn't wiggle or curve. It keeps the beam perfectly straight, explaining why the distant gamma rays look like a tight, focused jet rather than a messy cloud.
The Energy: The math works out that if the original explosion was powerful enough (a "PeV" event), the resulting neutrons would have just the right amount of energy to create the specific type of gamma rays we are seeing.

The "Amaterasu" Connection

The paper also suggests a fun side-note. The most energetic cosmic ray ever detected (named "Amaterasu") seems to come from a direction near SS433, but it's slightly off-center. The authors suggest that heavy atomic nuclei ejected during the same ancient explosion might have taken a curved path through the galaxy's magnetic fields, arriving at Earth slightly later and from a slightly different angle than the straight-line neutrons.

Summary

In short, the paper suggests that the mysterious, distant gamma-ray beams from SS433 are not a new explosion happening right now. Instead, they are the fossilized remains of an explosion that happened 100 years ago.

The black hole shot out a beam of invisible "ghost" neutrons. They flew straight through space for a century, undetected. Just recently, they reached the spot 75 light-years away, fell apart, and created a burst of electrons that lit up the sky in gamma rays, revealing the path of the invisible journey.

Technical Summary: PeV Neutrons as Origin of Separated SS433 TeV Signals

Problem Statement
The micro-quasar SS433, a binary system containing a black hole and a massive companion star, has long been known for its inner, precessing relativistic jets emitting radio, X-ray, and gamma-ray radiation. However, recent observations by H.E.S.S., HAWC, and LHAASO have revealed a puzzling phenomenon: a "twin" hard gamma-ray beam located tens of light-years (specifically 75 to 150 ly) away from the central source, completely disconnected from the inner jet. Existing models attempting to explain this via shock-wave re-acceleration at such distances struggle to account for the observed collimation of the beam. The paper addresses the challenge of explaining how TeV gamma-ray signals can appear so far from the source while maintaining a narrow, beam-like structure.

Methodology and Proposed Mechanism
The authors propose an alternative model based on high-energy nuclear physics, specifically the generation and decay of ultra-relativistic neutrons. The core hypothesis is that a rare, explosive flare (resembling a nova) occurred in the SS433 system approximately a century ago. During this event, a jet of tens of PeV ( $10^{16}$ eV) protons interacted with a dense thermal bath of ultraviolet photons emitted by the accretion disk.

The methodology involves the following physical chain:

Photo-nuclear Conversion: Tens of PeV protons undergo photo-nuclear interactions with UV photons (temperature $\approx 3.2 \times 10^5$ K) to form $\Delta^+$ resonances.
Neutron Production: The $\Delta^+$ resonance decays into a neutral pion and a neutron (or a proton and a charged pion). The neutral pion decays into gamma rays, while the neutron, being neutral, escapes the magnetic confinement of the source without deflection.
Relativistic Flight: A beam of neutrons with energies around 25–54 PeV travels unimpeded for decades.
Beta Decay in Flight: At a distance of roughly 75 to 150 light-years, these ultra-relativistic neutrons undergo beta decay ( $n \to p + e^- + \bar{\nu}_e$ ).
Secondary Emission: The decay produces tens of TeV electrons. These electrons subsequently generate the observed TeV gamma rays via Inverse Compton Scattering (ICS) off interstellar infrared photons.

The authors calculate the necessary conditions for this process, estimating the required flare luminosity ( $\sim 10^{40}$ erg/s) and photon density within the accretion disk to ensure a high probability of proton-to-neutron conversion. They also verify that the Larmor radii of the resulting charged secondaries (protons and electrons) are sufficiently small to maintain collimation over these distances, unlike the primary protons which would be deflected by galactic magnetic fields.

Key Contributions

Alternative Explanation for Disconnected Signals: The paper offers a mechanism that naturally explains the spatial separation and collimation of the TeV signals without requiring unexplained re-collimation mechanisms at vast distances.
Neutron as a "Courier": It identifies ultra-relativistic neutrons as ideal carriers of energy from the source to distant regions, as they are immune to magnetic deflection during their flight.
Quantitative Modeling: The authors provide specific calculations linking the observed 75–150 ly distance to the energy of the neutrons ( $\sim 25$ PeV) and the time elapsed since the hypothetical flare ( $\sim 100$ years), consistent with the neutron lifetime in the relativistic frame.
Connection to UHECR and GRBs: The model situates SS433 within the broader context of Ultra-High-Energy Cosmic Rays (UHECR) and Gamma-Ray Bursts (GRBs), suggesting that similar processes could explain the "Amaterasu" UHECR event and the nature of persistent jet acceleration in micro-quasars.

Results
The model successfully reproduces the observed distance of the TeV signals (75 ly for 25 PeV neutrons, up to 150 ly for higher energies) based on the relativistic time dilation of the neutron lifetime. The calculated probability of converting a proton jet into a neutron beam via the $\Delta^+$ resonance in the proposed UV photon bath is found to be high ( $\sigma \cdot n \cdot D \gg 1$ ), validating the feasibility of the mechanism. The resulting TeV gamma rays are attributed to the ICS of the decay electrons, matching the energy spectrum observed by current detectors.

Significance and Claims
The paper claims that the PeV neutron decay model provides a robust solution to the "enigmatic and sudden reappearing TeV tracks" of SS433. Its significance lies in:

Resolving the Collimation Paradox: Unlike shock-wave models, the neutron beam remains collimated naturally because neutral particles are not deflected by magnetic fields until they decay.
Historical Context: It posits that a specific, potentially undetected nova-like event occurred roughly a century ago (around the World War era), offering a testable hypothesis: a search for sudden luminosity variability in historical astronomical photographic plates from that epoch and direction.
Broader Implications: The authors suggest this mechanism could be a general feature of micro-quasars, potentially explaining neutrino spectra and energy discontinuities observed by IceCube, and offering a new perspective on the origin of specific UHECR events like Amaterasu.

The paper concludes that while the "tuned nebula mass density" required for alternative scattering models is prone to criticism, the neutron decay model relies on established nuclear physics and offers a more natural explanation for the observed separated twin TeV gamma beams.