Spectroscopy and polarimetry of the gravitationally lensed quasar Q0957+561

¹ Astronomical Observatory, Volgina 7, 11000 Belgrade, Serbia
e-mail: lpopovic@aob.rs
² Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, Belgrade 11000, Serbia
e-mail: lpopovic@matf.bg.ac.rs
³ Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz, Karachaevo-Cherkesia 369167, Russia
e-mail: vafan@sao.ru

Received: 15 November 2020
Accepted: 18 January 2021

Abstract

Context. We present new spectroscopic and polarimetric observations of the first discovered gravitational lens, Q0957+561. The lensed quasar has been observed with the 6 m telescope of the Special Astrophysical Observatory (Russia) in polarimetric and spectroscopic modes.

Aims. We explore the spectropolarimetric parameters of the A and B components of Q0957+561 to investigate the innermost structure of gravitationally lensed quasars and explore the nature of polarization in lensed quasars. Additionally, we aim to compare their present-day spectral characteristics with previous observations in order to study long-term spectral changes.

Methods. We perform new spectral and polarization observations of the Q0957+561 A and B images. After observed data reduction, we analyse the spectral characteristics of the lensed quasar, comparing the spectra of the A and B images, as well as comparing previously observed image spectra with present-day ones. The polarization parameters of the two images are also compared. Furthermore, we model the macro-lens influence on the polarization of the images, representing the gravitational lens with a singular isothermal elliptical potential.

Results. We find that the brightness and the spectral energy distribution ratio of components A and B have changed over a long period. Polarization in the broad lines of components A and B show that equatorial scattering cannot be detected in this lensed quasar. We find wavelength-dependent polarization that may be explained as a combination of the polarization from the disc and the outflowing material. There is a significant difference between the polarization parameters of the A and B images: The B component shows a higher polarization rate and polarization angle. However, both polarization vectors are nearly perpendicular to the observed radio jet projection. This indicates that the polarization in the continuum comes from the accretion disc. Our simple lensing model of a polarized source shows that, in principle, macro lenses can cause the observed differences in the polarization parameters of the Q0957+561A and B images. Using the Mg II broad line and luminosity of component A, we estimate the Q0957+561 black hole mass to be M_SMBH ≈ (4.8 − 6.1) × 10⁸ M_⊙.