Paper detail

Time Delays in the Gravitationally Lensed Quasar H1413+117 (Cloverleaf)

The quadruple quasar H1413+117 (z_s = 2.56) has been monitored with the 2.0 m Liverpool Telescope in the r Sloan band from 2008 February to July. This optical follow-up leads to accurate light curves of the four quasar images (A-D), which are defined by 33 epochs of observation and an average photometric error of \sim 15 mmag. We then use the observed (intrinsic) variations of \sim 50-100 mmag to measure the three time delays for the lens system for the first time (1σconfidence intervals): Δτ_{AB} = -17 +/- 3, Δτ_{AC} = -20 +/- 4, and Δτ_{AD} = 23 +/- 4 days (Δτ_{ij} = τ_j - τ_i; B and C are leading, while D is trailing). Although time delays for lens systems are often used to obtain the Hubble constant (H_0), the unavailability of the spectroscopic lens redshift (z_l) in the system H1413+117 prevents a determination of H_0 from the measured delays. In this paper, the new time delay constraints and a concordance expansion rate (H_0 = 70 km s^{-1} Mpc^{-1}) allow us to improve the lens model and to estimate the previously unknown z_l. Our 1σestimate z_l = 1.88^{+0.09}_{-0.11} is an example of how to infer the redshift of very distant galaxies via gravitational lensing.