Taking the full measure of gamma-ray halos – GAMALO

More than a decade of gamma-ray observations of the sky have revealed that pulsars transfer a significant fraction of their kinetic energy to ultrarelativistic electron-positron pairs that can eventually propagate out to the interstellar medium. Despite being energetically subdominant in cosmic rays compared to protons and heavy nuclei, electrons and positrons easily radiate their energy and will lighten up the environments of particle accelerators in our Galaxy, thereby contributing significantly to its overall gamma-ray emission, especially at very high energies.
The recent discovery of very extended gamma-ray halos around nearby pulsars has shown that leptons can be very efficiently confined around their sources, over large distances (>50 parsec) and long durations (>100000 years). How such a confinement is achieved remains uncertain. Possibilities include the injection of magnetic turbulence by kinetic instabilities from particles streaming away from the pulsar, resulting in a self-confinement of particles, and/or the injection of fluid turbulence on large scales by the supernova remnant expansion, followed by the cascading of turbulence to smaller scales where particle scattering can occur.
Whatever is the mechanism for confinement, if our Galaxy harbors a large population of thousands of gamma-ray halos there may be profound implications for the exploration and interpretation of the gamma-ray sky. This applies notably to the search of the sources of most energetic cosmic-ray nuclei, especially in star-forming regions and the innermost parts of our Galaxy where most pulsars reside. The existence of large and long-lived pair halos should therefore be included when trying to build an accurate picture of the gamma-ray emission of our Galaxy.
The goal of this project is to take the full measure of the gamma-ray halo phenomenon. By means of magnetohydrodynamic and particle-in-cell simulations we plan to explore the physics of pair confinement around pulsars to quantify if and how the phenomenon develops in a variety of galactic environments. Using existing gamma-ray observations we will probe the population of gamma-ray halos in our Galaxy over a broad energy range and at various scales, and firmly establish the place of these objects in the Galactic high-energy landscape. Last, based on this theoretical and observational knowledge, we will prepare for the observations of halos with forthcoming gamma-ray and radio instruments, developing tools and strategies for a further exploration of the phenomemon and to make sure that halos do not bias our intepretation of the measurements of other high-energy astrophysical phenonema.