COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses XII. Time delays of the doubly lensed quasars SDSS J1206+4332 and HS 2209+1914

TL;DR

This study presents long-term light curves and precise time-delay measurements for two doubly lensed quasars, using multi-telescope data and multiple analysis methods to improve accuracy and confirm previous results.

Contribution

It provides new time-delay estimates for SDSS J1206+4332 and HS 2209+1914 using extensive multi-year observations and diverse analysis techniques, enhancing the reliability of gravitational lensing measurements.

Findings

Confirmed previous time delay for SDSS J1206+4332 as 111.3 days

New time delay for HS 2209+1914 as 20.0 days

Achieved well-sampled light curves over nearly 9 seasons

Abstract

Aims. Within the framework of the COSMOGRAIL collaboration we present 7- and 8.5-year-long light curves and time-delay estimates for two gravitationally lensed quasars: SDSS J1206+4332 and HS 2209+1914. Methods. We monitored these doubly lensed quasars in the R-band using four telescopes: the Mercator, Maidanak, Himalayan Chandra, and Euler Telescopes, together spanning a period of 7 to 8.5 observing seasons from mid-2004 to mid-2011. The photometry of the quasar images was obtained through simultaneous deconvolution of these data. The time delays were determined from these resulting light curves using four very different techniques: a dispersion method, a spline fit, a regression difference technique, and a numerical model fit. This minimizes the bias that might be introduced by the use of a single method. Results. The time delay for SDSS J1206+4332 is Delta_t AB = 111.3 +/- 3 days with A leading B, confirming a previously published result within the error bars. For HS 2209+1914 we present a new time delay of Delta_t BA = 20.0 +/- 5 days with B leading A. Conclusions. The combination of data from up to four telescopes have led to well-sampled and nearly 9-season-long light curves, which were necessary to obtain these results, especially for the compact doubly lensed quasar HS 2209+1914.