Liverpool-Maidanak monitoring of the Einstein Cross in 2006–2019^⋆

I. Light curves in the gVrRI optical bands and microlensing signatures

¹ Departamento de Física Moderna, Universidad de Cantabria, Avda. de Los Castros s/n, E-39005 Santander, Spain
e-mail: goicol@unican.es
² Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetsky pr. 13, 119992 Moscow, Russia
e-mail: bartamon@mail.ru
³ O.Ya. Usikov Institute for Radiophysics and Electronics, National Academy of Sciences of Ukraine, 12 Acad. Proscury St., UA-61085 Kharkiv, Ukraine
e-mail: vshal@ukr.net
⁴ Institute of Astronomy of V.N. Karazin Kharkiv National University, Svobody Sq. 4, UA-61022 Kharkiv, Ukraine
⁵ Institute of Radio Astronomy of the National Academy of Sciences of Ukraine, 4 Mystetstv St., UA-61002 Kharkiv, Ukraine
⁶ Ulugh Beg Astronomical Institute of the Uzbek Academy of Sciences, Astronomicheskaya 33, 100052 Tashkent, Uzbekistan
⁷ National University of Uzbekistan, Department of Astronomy and Atmospheric Physics, 100174 Tashkent, Uzbekistan

Received: 6 March 2020
Accepted: 3 April 2020

Abstract

Quasar microlensing offers a unique opportunity to resolve tiny sources in distant active galactic nuclei and study compact object populations in lensing galaxies. We therefore searched for microlensing-induced variability of the gravitationally lensed quasar QSO 2237+0305 (Einstein Cross) using 4374 optical frames taken with the 2.0 m Liverpool Telescope and the 1.5 m Maidanak Telescope. These gVrRI frames over the 2006–2019 period were homogeneously processed to generate accurate long-term multi-band light curves of the four quasar images A–D. Through difference light curves, we found strong microlensing signatures. We then focused on the analytical modelling of two putative caustic-crossing events in image C, finding compelling evidence that this image experienced a double caustic crossing. Additionally, our overall results indicate that a standard accretion disc accounts reasonably well for the brightness profile of UV continuum emission sources and for the growth in source radius when the emission wavelength increases: R_λ ∝ λ^α, α = 1.33 ± 0.09. However, we caution that numerical microlensing simulations are required before firm conclusions can be reached on the UV emission scenario because the VRI-band monitoring during the first caustic crossing and one of our two α indicators lead to a few good solutions with α ≈ 1.