Using HAWC to Discover Invisible Pulsars
  Observations by HAWC and Milagro have detected bright and spatially extended
TeV gamma-ray sources surrounding the Geminga and Monogem pulsars. We argue
that these observations, along with a substantial population of other extended
TeV sources coincident with pulsar wind nebulae, constitute a new morphological
class of spatially extended TeV halos. We show that HAWCs wide field-of-view
unlocks an expansive parameter space of TeV halos not observable by atmospheric
Cherenkov telescopes. Under the assumption that Geminga and Monogem are typical
middle-aged pulsars, we show that ten-year HAWC observations should eventually
observe 37$^{+17}_{-13}$ middle-aged TeV halos that correspond to pulsars whose
radio emission is not beamed towards Earth. Depending on the extrapolation of
the TeV halo efficiency to young pulsars, HAWC could detect more than 100 TeV
halos from mis-aligned pulsars. These pulsars have historically been difficult
to detect with existing multiwavelength observations. TeV halos will constitute
a significant fraction of all HAWC sources, allowing follow-up observations to
efficiently find pulsar wind nebulae and thermal pulsar emission. The
observation and subsequent multi-wavelength follow-up of TeV halos will have
significant implications for our understanding of pulsar beam geometries, the
evolution of PWN, the diffusion of cosmic-rays near energetic pulsars, and the
contribution of pulsars to the cosmic-ray positron excess.
