The $>100$ kpc Distant Spur of the Sagittarius Stream and the Outer
  Virgo Overdensity, as Seen in PS1 RR Lyrae Stars
  We report the detection of spatially distinct stellar density features near
the apocenters of the Sagittarius (Sgr) stream's main leading and trailing arm.
These features are clearly visible in a high-fidelity stellar halo map that is
based on RR Lyrae from Pan-STARRS1: there is a plume of stars 10 kpc beyond the
apocenter of the leading arm, and there is a "spur" extending to 130 kpc,
almost 30 kpc beyond the previously detected apocenter of the trailing arm.
Such apocenter substructure is qualitatively expected in any Sgr stream model,
as stars stripped from the progenitor at different pericenter passages become
spatially separated there. The morphology of these new Sgr stream substructures
could provide much-needed new clues and constraints for modeling the Sgr
system, including the level of dynamical friction that Sgr has experienced. We
also report the discovery of a new, presumably unrelated halo substructure at
80 kpc from the Sun and $10^\circ$ from the Sgr orbital plane, which we dub the
Outer Virgo Overdensity.
