Revealing the structure of the lensed quasar Q 0957+561

I. Accretion disk size

¹ School of Physics and Astronomy and Wise Observatory, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv, Israel
e-mail: carinafian@mail.tau.ac.il
² Instituto de Astrofísica de Canarias, Vía Láctea s/n, La Laguna, 38200 Tenerife, Spain
³ Departamento de Astrofísica, Universidad de la Laguna, La Laguna, 38200 Tenerife, Spain
⁴ Departamento de Física Teórica y del Cosmos, Universidad de Granada, Campus de Fuentenueva, 18071 Granada, Spain
⁵ Instituto Carlos I de Física Teórica y Computacional, Universidad de Granada, 18071 Granada, Spain
⁶ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Playa Ancha, Valparaíso 2360102, Chile
⁷ Departamento de Astronomía y Astrofísica, Universidad de Valencia, 46100 Burjassot, Valencia, Spain
⁸ Observatorio Astronómico, Universidad de Valencia, 46980 Paterna, Valencia, Spain
⁹ Haifa Research Center for Theoretical Physics and Astrophysics, University of Haifa, Haifa, Israel
¹⁰ FRACTAL S.L.N.E., Calle Tulipán 2, Portal 13, 1A, 28231 Las Rozas de Madrid, Spain
¹¹ Institute of Physics, Laboratoire d’Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
¹² Institute of Physics (IGAM), University of Graz, Universitätsplatz 5, 8010 Graz, Austria

Received: 5 November 2020
Accepted: 6 July 2021

Abstract

Aims. We aim to use signatures of microlensing induced by stars in the foreground lens galaxy to infer the size of the accretion disk in the gravitationally lensed quasar Q 0957+561. The long-term photometric monitoring of this system (which so far has provided the longest available light curves of a gravitational lens system) permits us to evaluate the impact of uncertainties on our recently developed method (controlled by the distance between the modeled and the experimental magnitude difference histograms between two lensed images), and thus to test the robustness of microlensing-based disk-size estimates.

Methods. We analyzed the well-sampled 21-year GLENDAMA optical light curves of the double-lensed quasar and studied the intrinsic and extrinsic continuum variations. Using accurate measurements for the time delay between the images A and B, we modeled and removed the intrinsic quasar variability, and from the statistics of microlensing magnifications we used a Bayesian method to derive the size of the region emitting the continuum at λ_rest = 2558 Å.

Results. Analysis of the Q 0957+561 R-band light curves show a slow but systematic increase in the brightness of the B relative to the A component during the past ten years. The relatively low strength of the magnitude differences between the images indicates that the quasar has an unusually big optical accretion disk of half-light radius: R_1/2 = 17.6±6.1 √(M/0.3 M_⊙) lt-days.