A Triple Rollover: A third multiply-imaged source at z~6 behind the
  Jackpot gravitational lens
  UUsing a five-hour adaptive-optics-assisted observation with MUSE, we have
identified a doubly-imaged Ly alpha source at redshift 5.975 behind the z=0.222
lens galaxy J0946+1006 ('the Jackpot'). The source separation implies an
Einstein radius of ~2.5 arcsec. Combined with the two previously-known Einstein
rings in this lens (radii 1.4 arcsec at z = 0.609 and 2.1 arcsec at z ~ 2.4),
this system is now a unique galaxy-scale triple-source-plane lens. We show that
existing lensing models for J0946+1006 successfully map the two new observed
images to a common point on the z=5.975 source plane. The new source will
provide further constraints on the mass distribution in the lens and in the two
previously known sources. The third source also probes two new distance scaling
factors which are sensitive to the cosmological parameters of the Universe. We
show that detection of a new multiply imaged emission-line source is not
unexpected in observations of this depth; similar data for other known lenses
should reveal a larger sample of multiple-image-plane systems for cosmography
and other applications.
