COSMOGRAIL

XIX. Time delays in 18 strongly lensed quasars from 15 years of optical monitoring^⋆

¹ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
e-mail: martin.millon@epfl.ch
² Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
³ STAR Institute, Quartier Agora – Allée du six Août, 19c, 4000 Liège, Belgium
⁴ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁵ Observatoire Astronomique de l’Université de Genève, 51 ch. des Maillettes, 1290 Versoix, Switzerland
⁶ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
⁷ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
⁸ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

Received: 14 February 2020
Accepted: 8 June 2020

Abstract

We present the results of 15 years of monitoring lensed quasars, which was conducted by the COSMOGRAIL programme at the Leonhard Euler 1.2 m Swiss Telescope. The decade-long light curves of 23 lensed systems are presented for the first time. We complement our data set with other monitoring data available in the literature to measure the time delays in 18 systems, among which nine reach a relative precision better than 15% for at least one time delay. To achieve this, we developed an automated version of the curve-shifting toolbox PyCS to ensure robust estimation of the time delay in the presence of microlensing, while accounting for the errors due to the imperfect representation of microlensing. We also re-analysed the previously published time delays of RX J1131−1231 and HE 0435−1223, by adding six and two new seasons of monitoring, respectively, and confirming the previous time-delay measurements. When the time delay measurement is possible, we corrected the light curves of the lensed images from their time delay and present the difference curves to highlight the microlensing signal contained in the data. To date, this is the largest sample of decade-long lens monitoring data, which is useful to measure H₀ and the size of quasar accretion discs with microlensing as well as to study quasar variability.