Detection of a γ-ray halo around Geminga with the Fermi-LAT data and implications for the positron flux

The HAWC Collaboration has discovered a -ray emission extended about 2 degrees around the Geminga and Monogem pulsar wind nebulae (PWNe) at -ray energies E_>5 TeV. We analyze, for the first time, almost 10 years of -ray data obtained with the Fermi Large Area Telescope at E_>8 GeV in the direction of Geminga and Monogem. Since these two pulsars are close to the Galactic plane we run our analysis with ten different interstellar emission models (IEMs) to study the systematics due to the modeling of this component. We detect a -ray halo around Geminga with a significance in the range 7. 8--11. 8 depending on the IEM considered. This measurement is compatible with e^+ and e^- emitted by the PWN, which inverse Compton scatter (ICS) with photon fields located within a distance of about 100 pc from the pulsar, where the diffusion coefficient is estimated to be around 1. 110^27 cm^2/s at 100 GeV. We include in our analysis the proper motion of the Geminga pulsar which is relevant for rays produced for ICS in the Fermi-LAT energy range. We find that an efficiency of about 1% for the conversion of the spin-down energy of the pulsar into e^+ and e^- is required to be consistent with -ray data from Fermi-LAT and HAWC. The inferred contribution of Geminga to the e^+ flux is at most 20% at the highest-energy AMS-02 data. Our results are compatible with the interpretation that the cumulative emission from Galactic pulsars explains the positron excess.