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

An excess of cosmic positrons above 10 GeV with respect to the spallation reaction of cosmic rays (CRs) with the interstellar medium has been measured by Pamela, Fermi-LAT and with unprecedented precision by the AMS-02 experiment. Various interpretations have been invoked to interpret this excess, such as production from supernova remnants, pulsar wind nebulae (PWNe) and dark matter. A dominant contribution from dark matter is ruled out by the bounds found in gamma rays and other indirect searches. Models where supernova remnants produce secondary CRs struggle to explain the observed CR fluxes by AMS-02. Finally, severe constraints for a significant PWN contribution comes from the detection of very high-energy emission from Monogem and Geminga PWNe by Milagro and HAWC experiment. In this talk we will present a detailed study of the GeV gamma-ray halo around Geminga and Monogem, and show the constraints found for the contribution of these PWNe to the positron excess combining Milagro and HAWC data with measurements from the Fermi-LAT for the first time. We will report the detection of a significant emission from Geminga PWN derived by including the proper motion of the its pulsar. Then we will demonstrate that using gamma-ray data from the LAT is of central importance to provide a precise estimate for a PWN contribution to the cosmic positron flux. We will also present the most promising energies to detect a signal of synchrotron emission for positrons emitted by PWNe and will discuss the prospects of detecting this signature with current or future experiments.