Multi-GeV <i>Fermi</i> -LAT detection of PSR B1259-63

ABSTRACT PSR B1259–63/LS 2883 is a classical gamma-ray binary detected from radio to TeV energies near periastron. Using over 17 yr of Fermi-LAT observations, we report the first significant detection ($8\sigma$ in likelihood analysis) of the system in the 10–100 GeV energy range, over orbital phases from −400 to +100 d relative to periastron. The observed spectrum is well described by a power law with photon index $\Gamma = 1.9 \pm 0.1$ 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.