SS433 PeV neutron jet feeding the far TeV gamma beam

This paper proposes that the distant, disconnected TeV gamma-ray tails observed from the SS433 binary system originate from ultra-relativistic PeV neutron jets ejected during rare tidal eruptions, which subsequently decay and scatter to produce the observed high-energy radiation.

The Gist

The Mystery: A Ghostly Light Show

Imagine a cosmic lighthouse called SS433. It's a binary system where a massive black hole is eating a nearby star. As it eats, it shoots out two powerful jets of particles (like a garden hose spraying water) that spin around in a spiral, just like a rotating sprinkler. Astronomers have watched these jets for decades.

But recently, telescopes like H.E.S.S., HAWC, and LHAASO spotted something strange. About 75 to 150 light-years away from the lighthouse, there are bright, disconnected beams of high-energy gamma rays. It's as if you saw a fountain spraying water, and then suddenly, 100 miles away, a second fountain started spraying water out of thin air, with no hose connecting them.

Standard physics struggles to explain how a jet of particles could travel that far, stay perfectly straight (collimated), and then suddenly light up again without a visible source.

The Authors' Solution: The "Ghost Bullet" Theory

The authors, led by D. Fargion, propose a clever solution: The jet isn't made of charged particles; it's made of invisible "ghost bullets" (neutrons).

Here is the step-by-step story of their theory:

1. The "Explosive Party" (The Trigger)
About 100 years ago (around the time of World War I), the SS433 system had a massive, rare explosion—like a super-nova flare. During this party, the black hole shot out a beam of ultra-fast protons (charged particles) mixed with a bath of hot ultraviolet light.

2. The "Magic Trick" (The Conversion)
When these fast protons slammed into the ultraviolet light, they created a temporary, heavy particle called a Delta resonance. Think of this like a billiard ball hitting another ball and instantly splitting into two different balls.

• One ball was a proton (charged).
• The other ball was a neutron (neutral).

3. The "Invisible Runner" (The Journey)
This is the key part.

• Charged particles (like protons or electrons) are like magnets; they get pushed and pulled by magnetic fields in space, causing them to spiral and scatter. They can't travel in a straight line for 100 light-years.
• Neutrons are like ghosts. They have no electric charge, so magnetic fields ignore them. They fly in a perfectly straight line, undisturbed, for decades.

The authors suggest that 100 years ago, a beam of these "ghost neutrons" (carrying PeV-level energy) was shot out of SS433. They traveled silently through space, invisible to our telescopes, for nearly a century.

4. The "Sudden Reappearance" (The Decay)
Neutrons are unstable. They eventually decay (break apart) into a proton, an electron, and a neutrino.

• Because the neutrons were traveling so fast (near the speed of light), time slowed down for them, allowing them to survive the long journey.
• When they finally reached the spot 75 to 150 light-years away, they began to decay.
• The decay released high-energy electrons. These electrons then interacted with light to create the TeV gamma rays that the telescopes recently detected.

The Analogy:
Imagine a magician shooting a bullet from a gun. The bullet is invisible. It flies straight through a forest for 100 miles. Suddenly, at a specific spot, the bullet hits a target and explodes into a shower of colorful sparks. To an observer, it looks like the sparks appeared out of nowhere, 100 miles from the gun. The "bullet" was the neutron; the "sparks" are the gamma rays.

Why This Model Wins

The authors argue that other models (like shockwaves re-accelerating particles) have trouble explaining how the beam stays so straight and focused over such a huge distance. Magnetic fields would have scrambled a normal beam long ago. But a beam of neutral neutrons? It stays perfectly straight, like a laser beam, until it decays.

What This Means for Us

• The Timeline: The explosion that created this beam happened roughly a century ago. The authors suggest astronomers might find old photographic plates from that era showing a sudden brightening of SS433 that was missed at the time.
• The Connection: This theory links the distant gamma rays directly to a specific event in the past, solving the puzzle of why the light is so far away and disconnected.
• Neutrinos: The process also suggests that if we look for neutrinos (ghost particles) from this system, we should find them at specific energy levels (PeV range), which could help explain gaps in current neutrino data.

In short: The distant gamma rays aren't a new jet; they are the "footprints" of a beam of invisible neutrons that was shot out of SS433 a century ago and is only now breaking apart.

Technical Summary

Technical Summary: SS433 PeV Neutron Jet Feeding the Far TeV Gamma Beam

Problem Statement
The microquasar SS433, a binary system containing a black hole and a massive companion star, is known for its precessing, ultra-relativistic jets. While the inner jet structure is well-observed, recent detections by H.E.S.S., HAWC, and LHAASO have revealed a puzzling phenomenon: hard gamma-ray signatures (tens of TeV) located 75 to 150 light-years away from the central source. These distant emissions appear "disconnected" from the inner jet, forming collimated twin beams that defy standard models. Existing explanations relying on shock-wave re-acceleration and re-collimation of nuclei beams face significant challenges in explaining the beam's collimation over such vast distances.

Methodology and Proposed Mechanism
The authors propose an alternative model rooted in high-energy nuclear physics, specifically utilizing the interaction between ultra-high-energy (UHE) protons and thermal photons within the SS433 system. The core hypothesis suggests that approximately 75–150 years ago, the system underwent a rare, intense tidal eruption or nova-like flare. During this event, a jet of tens of PeV protons was ejected and interacted with a dense bath of ultraviolet (UV) thermal photons emitted by the accretion disk.

The mechanism proceeds through the following steps:

1. Delta Resonance Formation: Tens of PeV protons collide with UV photons (approx. 27.6 eV) to form $\Delta^+$ resonances via photo-nuclear scattering ($p + \gamma \to \Delta^+$).
2. Neutron Production: The $\Delta^+$ resonance decays, producing secondary beams of neutral pions and, crucially, ultra-relativistic neutrons with energies in the tens of PeV range.
3. Neutron Propagation: Unlike charged protons, these high-energy neutrons are neutral and travel undeflected by galactic magnetic fields. They maintain a collimated beam structure over distances of 75 to 150 light-years.
4. Beta Decay and Gamma Emission: Upon reaching the distant regions, the neutrons undergo in-flight beta decay ($n \to p + e^- + \bar{\nu}_e$). The resulting secondary electrons, carrying tens of TeV of energy, interact with ambient photons via Inverse Compton Scattering (ICS), re-emitting the observed tens of TeV gamma rays.

Key Calculations and Constraints
The paper validates the feasibility of this model through several physical constraints:

• Flight Distance: The decay length of a neutron is given by $L_n \approx 75 \times (E_n / 25 \text{ PeV})$ light-years. This aligns perfectly with the observed 75–150 light-year separation of the gamma-ray beams.
• Resonance Conditions: To produce 25 PeV neutrons, the model requires a proton energy of ~27.5 PeV interacting with photons of ~27.6 eV (UV range). This corresponds to a thermal temperature of roughly $3.2 \times 10^5$ K, achievable during a nova-like flare on the accretion disk.
• Conversion Probability: The authors estimate the photon number density in such a flare to be $1.7 \times 10^5$ times that of the solar spectrum. Using the $\Delta^+$ resonance cross-section ($\sigma_\Delta \approx 500 \mu b$), the optical depth ($\sigma_\Delta n_{Th} D_j$) is calculated to be significantly greater than 1 (approx. 23.2), indicating a high probability of efficient proton-to-neutron conversion.
• Larmor Radius: The model addresses why charged secondaries (protons and electrons) do not explain the distant beams. The Larmor radii for 25 PeV protons and 25 TeV electrons in the galactic magnetic field ($B_g \approx 3 \mu G$) are too small to allow collimated travel over 75 light-years. Only the neutral neutron beam can preserve the collimation required to explain the observations.

Results and Claims
The paper claims that the "neutron jet" model successfully explains:

• The disconnected nature of the TeV gamma-ray beams, as the neutrons travel invisibly before decaying.
• The collimation of the beams at large distances, which shock-reacceleration models struggle to maintain.
• The energy spectrum, linking the observed tens of TeV gamma rays to the beta decay of PeV neutrons.
• The timing, suggesting the initiating flare occurred roughly a century ago (potentially undetected due to the era's observational limitations or the system's later discovery in 1977).

The authors note that while alternative models involving gas cloud scattering exist, they require fine-tuned, ad-hoc density profiles that are less robust than the nuclear physics-based neutron decay mechanism.

Significance
The significance of this work lies in its ability to resolve the "enigmatic" and "sudden reappearing" TeV tracks of SS433 without invoking unexplained re-collimation mechanisms. By identifying a specific nuclear process (photo-pion production and neutron decay) analogous to the GZK cut-off mechanism in the cosmic background, the paper provides a coherent physical explanation for the distant emission. Furthermore, the authors suggest that this mechanism implies SS433 could be a source of Ultra-High-Energy Cosmic Rays (UHECRs) and specific neutrino signatures (PeV-scale), potentially linking the observed TeV gamma rays to critical energy windows in IceCube neutrino data. The paper concludes that this model offers a competitive, physically grounded explanation for the observed phenomena, inviting future observations by HAWC, H.E.S.S., and LHAASO to further validate or reject the neutron jet hypothesis.