A fully-spectroscopic triple-source-plane lens: the Jackpot completed

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