Speaker
Description
An excess in the flux of cosmic positrons at Earth above 10 GeV has been
measured by Pamela, Fermi-LAT and with unprecedented precision by AMS-02.
The observed flux cannot be explained by the production of positrons in
the spallation reaction of hadronic cosmic rays with the interstellar
medium. Various interpretations have been invoked to explain this excess,
such as the production in Galactic supernova remnants and pulsar wind
nebulae (PWNe) or, intriguinly, in the dark matter halo of the Milky Way.
Recently, Milagro and HAWC experiments reported the detection of an
extended gamma-ray emission from Geminga and Monogem PWNe at TeV energies.
These nearby and powerful PWNe have been widely considered as the main
candidates to contribute to the cosmic positrons at Earth. Severe
constraints for a significant PWNe contribution to the positron excess can
be derived from this gamma-ray emission, which has been interpreted as
coming from the electrons and positrons accelerated in the PWNe and
undergoing inverse Compton scattering in the interstellar medium.
Moreover, the size of extension of these halos suggests that the diffusion
around PWNe is about two orders of magnitude less intense than the value
assumed to fit the cosmic-rays measured by AMS-02. In this contribution we
report the first detection of a significant emission from the Geminga halo
at GeV energies in Fermi-LAT data, derived by including the proper motion
of its pulsar. We present a detailed study of the 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. The size of extension
and the consequences for the diffusion coefficient in these halos at GeV
energies are also explored. We demonstrate that using gamma-ray data from
the LAT is of central importance to provide a precise estimate for the PWN
contribution to the cosmic positron flux.
