A fully-spectroscopic triple-source-plane lens: the Jackpot completed
  We present a spectroscopic determination of the redshift of the second source
in the Jackpot gravitational lens system J0946+1006, for which only a
photometric estimate of $z_{\rm phot}$ = 2.41$^{+0.04}_{-0.21}$ has previously
been available. By visually inspecting an archival VLT X-Shooter observation,
we located a single emission line from the source in the H-band. Among the
possible options we find that this line is most likely to be [OIII] 5007 Ang at
$z_{\rm spec} $ = 2.035. Guided by this proposal, we were able to detect the
faint CIII] 1907,1909 Ang emission doublet in a deep VLT MUSE datacube. The
CIII] emission is spatially coincident with the brightest parts of the second
Einstein ring, and strongly supports the redshift identification. The
spectroscopic redshift is only marginally consistent with the photometric
estimate. Re-examining the cosmological constraints from J0946+1006, the
revised measurement favours less negative values of the dark energy
equation-of-state parameter $w$; when combined with a cosmic microwave
background prior, we infer $w$ = $-1.04\pm0.20$. The revised redshift does not
significantly help to reconcile the small discrepancy in the image positions
for the even more distant third source in J0946+1006.
