Self-Generated Cosmic-Ray Confinement in TeV Halos: Implications for TeV
  Gamma-Ray Emission and the Positron Excess
  Recent observations have detected extended TeV gamma-ray emission surrounding
young and middle-aged pulsars. The morphology of these "TeV halos" requires
cosmic-ray diffusion to be locally suppressed by a factor of ~100-1000 compared
to the typical interstellar medium. No model currently explains this
suppression. We show that cosmic-ray self-confinement can significantly inhibit
diffusion near pulsars. The steep cosmic-ray gradient generates Alfven waves
that resonantly scatter the same cosmic-ray population, suppressing diffusion
within ~20 pc of pulsars younger than ~100 kyr. In this model, TeV halos evolve
through two phases, a growth phase where Alfven waves are resonantly generated
and cosmic-ray diffusion becomes increasingly suppressed, and a subsequent
relaxation phase where the diffusion coefficient returns to the standard
interstellar value. Intriguingly, cosmic-rays are not strongly confined early
in the TeV halo evolution, allowing a significant fraction of injected e+e- to
escape. If these e+e- also escape from the surrounding supernova remnant, they
would provide a natural explanation for the positron excess observed by PAMELA
and AMS-02. Recently created TeV cosmic-rays are confined in the TeV halo,
matching observations by HAWC and H.E.S.S. While our default model relaxes too
rapidly to explain the confinement of TeV cosmic rays around mature pulsars,
such as Geminga, models utilizing a Kraichnan turbulence spectrum experience
much slower relaxation. Thus, observations of TeV halos around mature pulsars
may provide a probe into our understanding of interstellar turbulence.
