COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses XIII: Time delays and 9-yr optical monitoring of the lensed quasar RX J1131-1231

TL;DR

This nine-year optical monitoring study of the gravitationally lensed quasar RX J1131-1231 precisely measures time delays between its images, providing valuable data for cosmological applications and demonstrating robust analysis methods.

Contribution

The paper presents a comprehensive nine-year dataset of quasar light curves and introduces three consistent numerical techniques for accurate time delay measurement accounting for various observational uncertainties.

Findings

Delay between images A, B, C is compatible with zero.

Delay of image D is measured as 91 days with 1.5% uncertainty.

Analysis methods are robust across data subsets.

Abstract

We present the results from nine years of optically monitoring the gravitationally lensed z=0.658 quasar RX J1131-1231. The R-band light curves of the four individual images of the quasar were obtained using deconvolution photometry for a total of 707 epochs. Several sharp quasar variability features strongly constrain the time delays between the quasar images. Using three different numerical techniques, we measure these delays for all possible pairs of quasar images while always processing the four light curves simultaneously. For all three methods, the delays between the three close images A, B, and C are compatible with being 0, while we measure the delay of image D to be 91 days, with a fractional uncertainty of 1.5% (1 sigma), including systematic errors. Our analysis of random and systematic errors accounts in a realistic way for the observed quasar variability, fluctuating microlensing magnification over a broad range of temporal scales, noise properties, and seasonal gaps. Finally, we find that our time-delay measurement methods yield compatible results when applied to subsets of the data.