TeV Halos are Everywhere: Prospects for New Discoveries
  Milagro and HAWC have detected extended TeV gamma-ray emission around nearby
pulsar wind nebulae (PWNe). Building on these discoveries, Linden et al. [1]
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 CTA observations will detect
in total $\sim$50--240 TeV halos, though we note that multiple systematic
uncertainties still exist. Further, the existing HESS source catalog could
contain $\sim$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.
