Revealing the structure of the lensed quasar Q 0957+561

III. Constraints on the size of the broad-line region

¹ Departamento de Astronomía y Astrofísica, Universidad de Valencia, 46100 Burjassot, Valencia, Spain
e-mail: carina.fian@uv.es
² Observatorio Astronómico, Universidad de Valencia, 46980 Paterna, Valencia, Spain
³ Instituto de Astrofísica de Canarias, Vía Láctea S/N, La Laguna, 38200 Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de la Laguna, La Laguna, 38200 Tenerife, Spain
⁵ Departamento de Física Teórica y del Cosmos, Universidad de Granada, Campus de Fuentenueva, 18071 Granada, Spain
⁶ Instituto Carlos I de Física Teórica y Computacional, Universidad de Granada, 18071 Granada, Spain
⁷ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Playa Ancha, Valparaíso 2360102, Chile
⁸ Department of Physics, Faculty of Natural Sciences, University of Haifa, Haifa 3498838, Israel
⁹ Haifa Research Center for Theoretical Physics and Astrophysics, University of Haifa, Haifa 3498838, Israel
¹⁰ Institute of Physics (IGAM), University of Graz, Universitätsplatz 5, 8010 Graz, Austria
¹¹ Institute of Cosmology and Gravitation, University of Portsmouth, Burnaby Rd, Portsmouth PO1 3FX, UK

Received: 1 April 2021
Accepted: 17 June 2023

Abstract

Aims. Our aim is to examine the size, kinematics, and geometry of the broad-line region (BLR) in the double-lensed quasar Q 0957+561 by analyzing the impact of microlensing on various rest-frame ultraviolet broad-emission lines (BELs).

Methods. We explore the influence of intrinsic variability and microlensing on the C IV, C III], and Mg II emission lines through multiple spectroscopic observations taken between April 1999 and January 2017. By utilizing the line cores as a reference for no microlensing and correcting for the long time delay between the images, we estimate the sizes of the regions emitting the broad-line wings using a Bayesian approach.

Results. Our study of the microlensing amplitudes between the lensed images of the quasar Q 0957+561 reveals differing sizes of the regions emitting the three prominent BELs C IV, C III], and Mg II. The strength of the differential microlensing indicates that the high-ionization line C IV arises from a compact inner region of the BLR with a half-light radius of R_1/2 ≳ 16.0 lt-days, which represents a lower limit on the overall size of the BLR and is comparable to the size of the region emitting the r-band continuum in this system. A somewhat larger size of R_1/2 ≳ 44 lt-days is obtained for the semi-forbidden line C III]. Microlensing has a weak impact on the lower-ionization line Mg II, which is emitted from a region with a half-light radius of R_1/2 ≳ 50 lt-days. These findings suggest that the BEL regions may have distinct geometries and kinematics, with the more extended ones being spherically symmetric, and the most compact ones being nonspherical, with motions likely confined to a plane.