A Spectroscopically Confirmed Double Source Plane Lens System in the
  Hyper Suprime-Cam Subaru Strategic Program
  We report the serendipitous discovery of HSC J142449-005322, a double source
plane lens system in the Hyper Suprime-Cam Subaru Strategic Program. We dub the
system Eye of Horus. The lens galaxy is a very massive early-type galaxy with
stellar mass of ~7x10^11 Msun located at z_L=0.795. The system exhibits two
arcs/rings with clearly different colors, including several knots. We have
performed spectroscopic follow-up observations of the system with FIRE on
Magellan. The outer ring is confirmed at z_S2=1.988 with multiple emission
lines, while the inner arc and counterimage is confirmed at z_S1=1.302. This
makes it the first double source plane system with spectroscopic redshifts of
both sources. Interestingly, redshifts of two of the knots embedded in the
outer ring are found to be offset by delta_z=0.002 from the other knots,
suggesting that the outer ring consists of at least two distinct components in
the source plane. We perform lens modeling with two independent codes and
successfully reproduce the main features of the system. However, two of the
lensed sources separated by ~0.7 arcsec cannot be reproduced by a smooth
potential, and the addition of substructure to the lens potential is required
to reproduce them. Higher-resolution imaging of the system will help decipher
the origin of this lensing feature and potentially detect the substructure.
