STREAMFINDER II: A possible fanning structure parallel to the GD-1
  stream in Pan-STARRS1
  STREAMFINDER is a new algorithm that we have built to detect stellar streams
in an automated and systematic way in astrophysical datasets that possess any
combination of positional and kinematic information. In Paper I, we introduced
the methodology and the workings of our algorithm and showed that it is capable
of detecting ultra-faint and distant halo stream structures containing as few
as $\sim 15$ members ($\Sigma_{\rm G} \sim 33.6\, {\rm mag \, arcsec^{-2}}$) in
the Gaia dataset. Here, we test the method with real proper motion data from
the Pan-STARRS1 survey, and by selecting targets down to $r_{0}=18.5$ mag we
show that it is able to detect the GD-1 stellar stream, whereas the structure
remains below a useful detection limit when using a Matched Filter technique.
The radial velocity solutions provided by STREAMFINDER for GD-1 candidate
members are found to be in good agreement with observations. Furthermore, our
algorithm detects a $\sim 40^{\deg}$ long structure approximately parallel to
GD-1, and which fans out from it, possibly a sign of stream-fanning due to the
triaxiality of the Galactic potential. This analysis shows the promise of this
method for detecting and analysing stellar streams in the upcoming Gaia DR2
catalogue.
