TeV halos are everywhere: Prospects for new discoveries

Milagro and the High Altitude Water Cherenkov (HAWC) observatory have detected extended TeV gamma-ray emission around nearby pulsar wind nebulae. Building on these discoveries, T. Linden et al., Phys. Rev. D 96, 103016 (2017). identified a new source class-TeV halos-powered by the interactions of high-energy electrons and positrons that have escaped from the PWN, but which remain trapped in a larger region where diffusion is inhibited compared to the interstellar medium. Many theoretical properties of TeV halos remain mysterious, but empirical arguments suggest that they are ubiquitous. The key to progress is finding more halos. We outline prospects for new discoveries and calculate their expectations and uncertainties. We predict, using models normalized to current data, that future HAWC and Cherenkov Telescope Array observations will detect in total ∼50–240 TeV halos, though we note that multiple systematic uncertainties still exist. Further, the existing High Energy Stereoscopic System source catalog could contain ∼10–50 TeV halos that are presently classified as unidentified sources or PWN candidates. We quantify the importance of these detections for new probes of the evolution of TeV halos, pulsar properties, and the sources of high-energy gamma rays and cosmic rays.