🔭 astrophysics

Prospects for Observing the Microquasar SS 433 with the LACT Array

Through detailed simulations, this paper demonstrates that the upcoming LACT Cherenkov telescope array can detect the microquasar SS 433 with high significance, resolve its jets, and distinguish between competing spectral models, thereby offering critical insights into particle acceleration and radiation mechanisms in PeVatrons.

Original authors: Zhen Xie, Zhipeng Zhang, Ruizhi Yang

Published 2026-03-25

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CC BY 4.0

Original authors: Zhen Xie, Zhipeng Zhang, Ruizhi Yang

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 construction site. In the middle of this site, there's a tiny, incredibly powerful machine called SS 433. It's a "microquasar"—basically a black hole or neutron star that is greedily eating its neighbor star. As it eats, it spits out two massive, high-speed jets of particles, like a garden hose turned up to maximum pressure, but shooting out at 26% the speed of light.

For decades, astronomers have tried to figure out exactly how these jets work. Are they powered by magnetic fields (like a lightning storm)? Or are they powered by smashing protons together (like a particle collider)? The problem is, these jets are far away, and the "cameras" we've used so far are either too blurry to see the details or not sensitive enough to catch the faintest signals.

Enter the LACT Array. Think of LACT as the new, super-powered "night-vision goggles" for the universe. It's a brand-new telescope system currently being built right next to the famous LHAASO observatory in China.

Here is what this paper says LACT can do, explained simply:

1. The "Super-Sharp" Vision

Imagine trying to read the license plate of a car driving on the other side of a city. Old telescopes (like H.E.S.S.) could tell you a car was there, but the image was blurry. LHAASO could see the car clearly but only from a distance.

LACT is like having a high-definition camera with a massive zoom lens. The paper's simulations show that with just 30 hours of looking at SS 433, LACT will be able to clearly separate the "East" jet from the "West" jet. It's like finally being able to see that the two garden hoses are actually spraying in slightly different directions, rather than just seeing one giant, blurry mist.

2. The "Time Machine" for Particles

These jets are like cosmic fireworks. Sometimes they flare up, sometimes they fade. The paper suggests that LACT can catch these fireworks in high definition.

The Goal: They want to see where the particles are being accelerated. Are they speeding up right at the base of the jet (near the black hole), or further out where the jet hits gas clouds?
The Result: LACT is so sensitive that it can map out these "explosion zones" much better than current tools. It's like going from seeing a blurry photo of a car crash to seeing a slow-motion video that shows exactly which part of the car hit first.

3. The "Detective Work": Who is the Culprit?

This is the most exciting part. Astronomers are arguing about what makes the gamma rays (the highest energy light) from SS 433.

Theory A (The Leptons): It's just electrons zooming around and glowing.
Theory B (The Hadrons): It's protons (heavy particles) smashing into gas clouds, creating a cosmic "smoke" of gamma rays.

The paper suggests that LACT can act as a detective to solve this mystery. By looking at the energy of the light, LACT can tell the difference.

If they look at the central region of SS 433 for about 100 hours, LACT might finally see the "smoke" of the proton collisions (the hadronic component).
It's like trying to hear a whisper in a noisy room. Previous telescopes were too far away or too noisy to hear the whisper. LACT is like a parabolic microphone that can isolate that specific whisper from the background noise.

4. The "Night Shift" Advantage

One cool thing about LACT is its design. Most telescopes can only work on very dark, moonless nights. LACT is built with special cameras that can work even when the moon is out. This means it can work more hours, like a construction crew that doesn't have to stop just because the sun came up (or the moon came out). This gives them more time to catch those rare, high-energy events.

The Bottom Line

This paper is essentially a "proof of concept" or a "blueprint." It says: "If we build LACT and point it at SS 433, here is exactly what we will see."

The answer is: We will finally understand the engine.
LACT will allow us to see the structure of these cosmic jets with crystal clarity, figure out if they are powered by electrons or protons, and help us understand how the universe accelerates particles to energies we can't even imagine on Earth. It's the next big step in unlocking the secrets of our galaxy's most violent machines.

1. Problem Statement

The microquasar SS 433 is a critical laboratory for studying relativistic jet physics, particle acceleration, and non-thermal emission mechanisms. While previous observatories (HAWC, H.E.S.S., and LHAASO) have detected very-high-energy (VHE) and ultra-high-energy (UHE) gamma-ray emission from SS 433, significant gaps remain in understanding the system:

Morphological Resolution: LHAASO has detected extended UHE emission (>100 TeV) from the central region, suggesting a hadronic origin, but its angular resolution is insufficient to resolve the fine spatial structure of the jets or separate the central component from the jet bases.
Spectral Sensitivity: H.E.S.S. offers superior angular resolution but lacks the sensitivity required for systematic exploration in the "tens-of-TeV" energy range (10–100 TeV), a critical band for distinguishing between leptonic and hadronic emission components.
Unresolved Physics: Key questions regarding the physical origin of jet re-brightening, the dominant radiative processes, and the spatial separation of leptonic (jet) and hadronic (central cloud) components remain unanswered due to instrument limitations.

2. Methodology

The authors employed detailed Monte Carlo simulations to evaluate the observational capabilities of the upcoming Large Array of imaging atmospheric Cherenkov Telescopes (LACT), located at the LHAASO site (4400 m altitude).

Instrument Configuration: The study utilized Instrument Response Functions (IRFs) for LACT (32 telescopes, 6m diameter, SiPM cameras) under two observation modes:
- Near-Zenith (20°): Standard mode for high sensitivity.
- Large-Zenith-Angle (LZA, 60°): Optimized for UHE events and extended sky coverage.
Source Modeling:
- Jet Components: Modeled based on H.E.S.S. observations (spatially symmetric Gaussian for east/west jets, spectral indices $\Gamma \approx 2.2\text{--}2.4$ ).
- Central Component: Modeled based on LHAASO findings (LHAASO J1911+0513), representing a potential hadronic component with a steep spectrum ( $\Gamma \approx 3.96$ ) above 100 TeV.
- Three-Source Model: A composite model including the two leptonic jets and the central hadronic source was used to test LACT's ability to disentangle emission mechanisms.
Simulation Framework:
- Used the Gammapy-1.3 analysis framework extended with LACT-specific IRFs.
- Simulated event lists for energy bands of 10–300 TeV.
- Performed 100 independent realizations for each exposure time to ensure statistical robustness.
- Calculated detection significance ($TS$) and reconstructed spatial/spectral characteristics.
Observation Strategy: Estimated effective observation times from the LHAASO site (October–April) for zenith angles $<50^\circ$ (127.8 hours total) and $<70^\circ$ (330.3 hours total), accounting for moonless nights.

3. Key Contributions

Quantitative Feasibility Study: Provided the first detailed simulation of LACT's performance specifically for SS 433, establishing a baseline for future observational campaigns.
Differentiation of Emission Mechanisms: Demonstrated LACT's unique capability to spatially separate the leptonic jet emission from the central hadronic component, a task currently impossible for LHAASO (due to resolution) and H.E.S.S. (due to sensitivity at high energies).
Optimization of Observation Modes: Evaluated the trade-offs between near-zenith and large-zenith-angle observations, showing how LZA modes can extend sensitivity to the highest energies (UHE) despite lower duty cycles.
Synergy Proposal: Proposed a joint analysis strategy combining LACT (Cherenkov light) and LHAASO-KM2A (particle detection/Muon detectors) to achieve background-free selection and enhanced statistics for UHE events.

4. Key Results

Jet Detection and Resolution:
- LACT can achieve a $5\sigma$ detection significance for the eastern and western jets with approximately 30 hours of observation (10–300 TeV range).
- This is significantly faster than current H.E.S.S. capabilities (which required ~200 hours for similar significance).
- LACT's superior angular resolution allows for the clear spatial separation of the east and west jets.
Central Hadronic Component:
- LACT can marginally detect ( $3\sigma$ ) the central hadronic component after ~50 hours of observation with an energy threshold of 30 TeV.
- For a threshold of 50 TeV, approximately 100 hours of exposure are required to reach $3\sigma$ .
- Detection above 100 TeV is photon-statistics limited, requiring joint analysis with LHAASO-KM2A for significant results.
Spectral Model Discrimination:
- Simulations indicate LACT can distinguish between the H.E.S.S. and LHAASO spectral models, particularly in the tens-of-TeV range where the transition between leptonic and hadronic dominance is expected.
- The difference in significance between model fits increases with exposure time, though the steep spectral cutoff (Klein-Nishina effect) may limit significance at energies $>100$ TeV.
Morphological Evolution: The simulations predict that LACT will resolve the energy-dependent morphology, showing confinement of photons $>10$ TeV to the jet bases and extended emission at lower energies.

5. Significance

Advancing PeVatron Physics: SS 433 is a prime candidate for a Galactic PeVatron. LACT's ability to resolve the spatial distribution of acceleration sites and distinguish between leptonic and hadronic processes will provide critical constraints on particle escape, diffusion, and cooling mechanisms.
Bridging the Energy Gap: LACT fills the critical sensitivity gap in the "tens-of-TeV" band, a regime where current instruments struggle to provide systematic data. This is essential for understanding the spectral breaks and the transition between emission components.
Future Observational Roadmap: The study provides a quantitative basis for optimizing exposure strategies and observation modes (zenith angles) for the upcoming LACT commissioning phase.
Broader Impact: The simulation framework and analysis techniques developed for SS 433 are applicable to a wide range of Galactic and extragalactic gamma-ray sources, promising new insights into particle acceleration in extreme astrophysical environments.

In conclusion, the paper establishes that the LACT array will be a transformative instrument for high-resolution gamma-ray astronomy, capable of solving long-standing mysteries regarding the multi-wavelength emission mechanisms of microquasars like SS 433.