Multi-GeV Fermi-LAT Detection of PSR B1259-63

PSR B1259−63/LS 2883is a classical gamma-ray binary detected from radio to TeV energies near periastron. Using over 17 years of Fermi-LAT observations, we report the first significant detection (8σ in likelihood analysis) of the system in the 10-100 GeV energy range, over orbital phases from -400 to +100 days relative to periastron. The observed spectrum is well described by a power law with photon index Γ = 1.9 ± 0.1 and shows a flux level consistent with that measured at TeV energies by H.E.S.S. The smooth connection between the Fermi-LAT and TeV spectra suggests that the detected multi-GeV emission traces the rising tail of the inverse-Compton component extending into the TeV regime. The presence of detectable emission hundreds of days before periastron indicates high-energy activity over a larger orbital phase range than previously established, enabling new constraints on particle-acceleration and radiative processes in the system.