Detection of a $\gamma$-ray halo around Geminga with the $Fermi$-LAT and implications for the positron flux
July 22, 2019
An excess of cosmic-ray 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 AMS-02.
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 come from the detection of very high-energy gamma-ray emission from Monogem and Geminga PWNe by Milagro and HAWC experiments.
In this contribution we 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 cosmic-ray positron excess, combining Milagro and HAWC data with measurements from the Fermi-LAT for the first time. We report the detection of a significant emission from Geminga PWN, derived by including the proper motion of its pulsar. We 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
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