Gravitationally lensed quasars can be used to map the mass distribution in lensing galaxies and to estimate the Hubble constant H _ { 0 } by measuring the time delays between the quasar images .
Here we report the measurement of two independent time delays in the quadruply imaged quasar WFI J2033–4723 ( z = 1.66 ) .
Our data consist of R -band images obtained with the Swiss 1.2 m EULER telescope located at La Silla and with the 1.3 m SMARTS telescope located at Cerro Tololo .
The light curves have 218 independent epochs spanning 3 full years of monitoring between March 2004 and May 2007 , with a mean temporal sampling of one observation every 4th day .
We measure the time delays using three different techniques , and we obtain \Delta t _ { B - A } = 35.5 \pm 1.4 days ( 3.8 % ) and \Delta t _ { B - C } = 62.6 ~ { } ^ { + 4.1 } _ { - 2.3 } ~ { } \hbox { days } ~ { } ( ^ { + 6.5 \% } _ { - % 3.7 \% } ) , where A is a composite of the close , merging image pair .
After correcting for the time delays , we find R -band flux ratios of F _ { A } / F _ { B } = 2.88 \pm 0.04 , F _ { A } / F _ { C } = 3.38 \pm 0.06 , and F _ { A 1 } / F _ { A 2 } = 1.37 \pm 0.05 with no evidence for microlensing variability over a time scale of three years .
However , these flux ratios do not agree with those measured in the quasar emission lines , suggesting that longer term microlensing is present .
Our estimate of H _ { 0 } agrees with the concordance value : non-parametric modeling of the lensing galaxy predicts H _ { 0 } = 67 ^ { +13 } _ { -10 } km s ^ { -1 } Mpc ^ { -1 } , while the Single Isothermal Sphere model yields H _ { 0 } = 63 ^ { +7 } _ { -3 } km s ^ { -1 } Mpc ^ { -1 } ( 68 % confidence level ) .
More complex lens models using a composite de Vaucouleurs plus NFW galaxy mass profile show twisting of the mass isocontours in the lensing galaxy , as do the non-parametric models .
As all models also require a significant external shear , this suggests that the lens is a member of the group of galaxies seen in field of view of WFI J2033–4723 .