COSMOGRAIL

XVII. Time delays for the quadruply imaged quasar PG 1115+080^⋆

¹ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
e-mail: vivien.bonvin@epfl.ch
² Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Playa Ancha, Valparaíso 2360102, Chile
³ Departamento de Ciencias Físicas, Universidad Andres Bello Fernandez Concha 700, Las Condes, Santiago, Chile
⁴ Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Macul 7820436, Santiago, Chile
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Department of Physics, University of California, Davis, CA 95616, USA
⁷ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁸ Fermi National Accelerator Laboratory, PO Box 500, Batavia, IL 60510, USA
⁹ Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
¹⁰ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94035, USA
¹¹ Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85740 Garching, Germany
¹² Physik-Department, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany
¹³ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 10617, Taiwan
¹⁴ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
¹⁵ Millennium Institute of Astrophysics, Chile
¹⁶ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, allée du 6 Août 17, 4000 Liège, Belgium

Received: 24 April 2018
Accepted: 31 May 2018

Abstract

We present time-delay estimates for the quadruply imaged quasar PG 1115+080. Our results are based on almost daily observations for seven months at the ESO MPIA 2.2 m telescope at La Silla Observatory, reaching a signal-to-noise ratio of about 1000 per quasar image. In addition, we re-analyze existing light curves from the literature that we complete with an additional three seasons of monitoring with the Mercator telescope at La Palma Observatory. When exploring the possible source of bias we considered the so-called microlensing time delay, a potential source of systematic error so far never directly accounted for in previous time-delay publications. In 15 yr of data on PG 1115+080, we find no strong evidence of microlensing time delay. Therefore not accounting for this effect, our time-delay estimates on the individual data sets are in good agreement with each other and with the literature. Combining the data sets, we obtain the most precise time-delay estimates to date on PG 1115+080, with Δt(AB) = 8.3^+1.5_−1.6 days (18.7% precision), Δt(AC) = 9.9^+1.1_−1.1 days (11.1%) and Δt(BC) = 18.8^+1.6_−1.6 days (8.5%). Turning these time delays into cosmological constraints is done in a companion paper that makes use of ground-based Adaptive Optics (AO) with the Keck telescope.