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First Detection of Ultrahigh Energy Emission from Gamma-Ray Binary LS I +61° 303
Zhen Cao1,2,3, F Aharonian3,4,5,6, Y X Bai1,3
1Key Laboratory of Particle Astrophysics and Experimental Physics Division and Computing Center, Institute of High Energy Physics, Chinese Academy of Sciences, 100049 Beijing, China.
Physical Review Letters
|May 22, 2026
Summary
The Large High Altitude Air Shower Observatory detected ultrahigh-energy gamma-ray emission from the LS I +61° 303 binary system. This finding provides evidence for extreme particle acceleration and shows energy-dependent orbital modulation.
Area of Science:
- High-energy astrophysics
- Gamma-ray astronomy
- Particle physics
Background:
- LS I +61° 303 is a well-known gamma-ray binary system.
- Previous observations have detected gamma-ray emission from this source, but not at ultrahigh energies.
Purpose of the Study:
- To report the first detection of ultrahigh-energy (UHE) gamma-ray emission from LS I +61° 303.
- To investigate the particle acceleration mechanisms and orbital modulation in this system.
Main Methods:
- Utilized data from the Large High Altitude Air Shower Observatory (LHAASO).
- Analyzed data from the Water Cherenkov Detector Array (WCDA) and the Kilometer Square Array (KM2A).
- Searched for UHE photonlike events above 100 TeV.
Main Results:
- Confirmed UHE gamma-ray emission from LS I +61° 303 with high statistical significance (9.2σ in WCDA, 6.2σ in KM2A).
- Detected 16 UHE photonlike events above 100 TeV in KM2A data.
- Observed orbital modulation in the 25-100 TeV range with a 3.9σ significance, suggesting energy dependence.
Conclusions:
- The results provide compelling evidence for extreme particle acceleration in LS I +61° 303.
- A composite scenario involving both leptonic and hadronic processes is proposed to explain the observed features.
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