Liverpool-Maidanak monitoring of the Einstein Cross in 2006–2019
L. J. Goicoechea, Б. П. Артамонов, V. N. Shalyapin, A. V. Sergeyev, О. Бурхонов, Talat Akhunov, I. M. Asfandiyarov, В. В. Бруевич, Sh. A. Ehgamberdiev, E. V. Shimanovskaya, Alexander Zheleznyak
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 g V r R I 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 V R I -band monitoring during the first caustic crossing and one of our two α indicators lead to a few good solutions with α ≈ 1.