Spectroscopy and polarimetry of the gravitationally lensed quasar Q0957+561

TL;DR

This study provides new spectroscopic and polarimetric data on the gravitationally lensed quasar Q0957+561, revealing polarization differences between images and modeling the lens's effect on polarization, along with long-term spectral change analysis.

Contribution

It offers the first detailed spectropolarimetric analysis of Q0957+561, including modeling of macro-lens influence on polarization and long-term spectral comparison, advancing understanding of quasar inner structures and lens effects.

Findings

Polarization in broad lines indicates no equatorial scattering.

Wavelength-dependent polarization suggests disc and outflow contributions.

Macro-lens can account for polarization differences between images.

Abstract

We present new spectroscopic and polarimetric observations of the first discovered gravitational lens Q0957+561 obtained with the 6m telescope of the Special Astrophysical Observatory (SAO, Russia). We explore spectropolarimetric parameters of Q0957+561 A,B components to investigate the innermost structure of the quasar, and explore the nature of polarization in lensed quasars. Additionally, we compare their present-day spectral characteristics with previous observations in order to study long-term spectral changes. We analyze spectral characteristics of lensed quasar comparing spectra of A and B images, as well as comparing previously observed image spectra with present-day ones. The polarization parameters of A-B images are compared. We also model the macro-lens influence on the polarization of the images representing the gravitational lens with a singular isothermal elliptical potential. We find that the brightness and SED ratio of components A and B changed during a long period. Polarization in broad lines of components A and B showed that the equatorial scattering cannot be detected in this quasar. We find wavelength-dependent polarization that may be explained as a combination of the polarization from the disc and outflowing material. There is a significant difference between polarization parameters of the A and B images: the B component shows a higher polarization degree and polarization angle. However, both polarization vectors are nearly perpendicular to the observed radio jet projection. It indicates that the polarization in the continuum is coming from the accretion disc. Our simple lensing model of a polarized source showed that macro-lens can cause the observed differences in polarization parameters of Q0957+561 A,B images. Using Mg II broad line and luminosity of component A we estimated that the Q0957+561 black hole mass is M~(4.8-6.1) $10^8$ M$\odot$