Detection of Strong Epicyclic Density Spikes in the GD-1 Stellar Stream.
  An Absence of Evidence for the Influence of Dark Matter Subhalos?
  The density variations in thin stellar streams may encode important
information on the nature of the dark matter. For instance, if dark matter
aggregates into massive sub-halos, these perturbers are expected to scatter
stars out of dynamically cold stellar streams, possibly leading to detectable
gaps in those structures. Here we re-examine the density variations in the GD-1
stream, using Gaia DR2 astrometry, Pan-STARRS photometry, together with high
precision radial velocities measured with the CFHT/ESPaDOnS and VLT/UVES
instruments and complemented with public radial velocity catalogs. We show that
after correcting for projection effects, the density profile exhibits high
contrast periodic peaks, separated by $2.64\pm0.18$ kpc. An N-body simulation
is presented that reproduces this striking morphology with simple epicyclic
motion in a smooth Galactic potential. We also discuss the reliability of
measuring density variations using ground-based photometric surveys, and for
the particular case of GD-1 we highlight some of the artifacts present in the
Gaia DR2 catalog along its track. Massive dark subhalos do not appear to be
required to explain the density clumping along GD-1.
