First detection of ultra-high energy emission from gamma-ray binary LS I +61 303

Abstract: We report the first detection of gamma-ray emission up to ultra-high-energy (UHE; $>$100 TeV) emission from the prototypical gamma-ray binary system LS I +61 303 using data from the Large High Altitude Air Shower Observatory (LHAASO). It is detected with significances of 9.2$\sigma$ in WCDA (1.4--30.5 TeV) and 6.2$\sigma$ in KM2A (25--267 TeV); in KM2A alone we identify 16 photon-like events above 100 TeV against an estimated 5.1 background events, corresponding to a 3.8$\sigma$ detection. These results provide compelling evidence of extreme particle acceleration in LS I +61 303. Furthermore, we observe orbital modulation at 4.0$\sigma$ confidence between 25 and 100 TeV, and a hint of energy-dependent orbital modulation. These features can be understood in a composite scenario in which leptonic and hadronic processes jointly contribute.