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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.
Abstract:
We report the first detection of gamma-ray emission up to ultrahigh-energy (UHE; >100 TeV) emission from the prototypical gamma-ray binary system LS I +61° 303 using data from the Large High Altitude Air Shower Observatory. It is detected with significances of 9.2σ in the Water Cherenkov Detector Array (WCDA, 1.4-30.5 TeV) and 6.2σ in the Kilometer Square Array (KM2A, 25-267 TeV); in KM2A alone we identify 16 photonlike events above 100 TeV against an estimated 5.1 background events, corresponding to a 3.8σ detection. These results provide compelling evidence of extreme particle acceleration in LS I +61° 303. Furthermore, we observe orbital modulation at 3.9σ confidence level, between 25 and 100 TeV, with a hint that the orbital modulation is energy dependent. These features can be understood in a composite scenario in which leptonic and hadronic processes jointly contribute.
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