This paper presents optical R-band light curves and the time delay of the doubly imaged gravitationally lensed quasar SDSS J1001 + 5027 at a redshift of 1.838 .
We have observed this target for more than six years , between March 2005 and July 2011 , using the 1.2-m Mercator Telescope , the 1.5-m telescope of the Maidanak Observatory , and the 2-m Himalayan Chandra Telescope .
Our resulting light curves are composed of 443 independent epochs , and show strong intrinsic quasar variability , with an amplitude of the order of 0.2 magnitudes .
From this data , we measure the time delay using five different methods , all relying on distinct approaches .
One of these techniques is a new development presented in this paper .
All our time-delay measurements are perfectly compatible .
By combining them , we conclude that image A is leading B by 119.3 \pm 3.3 days ( 1 \sigma , 2.8 % uncertainty ) , including systematic errors .
It has been shown recently that such accurate time-delay measurements offer a highly complementary probe of dark energy and spatial curvature , as they independently constrain the Hubble constant .
The next mandatory step towards using SDSS J1001 + 5027 in this context will be the measurement of the velocity dispersion of the lensing galaxy , in combination with deep Hubble Space Telescope imaging .