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

TL;DR

This paper reports the first detection of ultra-high-energy gamma-ray emission from the binary system LS I +61 303, revealing extreme particle acceleration and orbital modulation at energies above 100 TeV.

Contribution

It presents the first observation of UHE gamma-ray emission from LS I +61 303 using LHAASO data, demonstrating extreme acceleration and orbital modulation evidence.

Findings

Detection of gamma rays >100 TeV with high significance

Observation of orbital modulation at 25-100 TeV

Evidence for combined leptonic and hadronic processes

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 3.9$\sigma$ 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.