Quasars' Optical Polarization and Balmer Edge Feature Revealed by Ultra-Violet, and Polarized Visible to Near Infrared Emissions

TL;DR

This study demonstrates that polarized emission analysis from quasars reveals the intrinsic accretion disc spectrum, including the broad Balmer edge feature, enabling estimation of the quasar's inclination angle.

Contribution

It introduces a method to determine quasar inclination by modeling polarized spectra with an extended accretion disc model including Balmer edge absorption.

Findings

The extended thin disc model fits UV to NIR spectra well.

Polarization measurements constrain the true disc polarization fraction.

The broadening of the Balmer edge relates to the quasar's inclination.

Abstract

Polarized emission from a quasar is produced by wavelength-independent electron scattering surrounding its accretion disc, and thus avoid the contamination from its host galaxy and reveal the intrinsic emission spectrum of the accretion disc. Ultra-violet (UV) emission from a quasar is normally free from the contamination from its host galaxy. Polarization fraction of the quasar's disc emission can therefore be determined by comparing total UV emission with polarized visible to near-infrared (NIR) emission; and the resulting continuum spectrum from UV to infrared can reveal the theoretically expected Balmer edge absorption feature. We fit the polarized spectra in visible and NIR bands together with the total UV spectra of two type-1 quasars (3C 95, 4C 09.72), to an extended geometrically thin and optically thick accretion disc model. In addition to the standard model, we include the Balmer edge absorption due to co-rotational neutral gas on a narrow annulus of the accretion disc. We find that the extended thin accretion disc model provides adequate description on the continuum spectra of the two quasars from UV to NIR wavelengths. A Monte-Carlo-Markov-Chain fitting to the continuum spectra is able to well constrain the true polarization fraction of the disk emission, which allows the Balmer edge feature to be completely revealed from polarized visible to UV continua. The Balmer edge feature is prominent in both quasars' spectra, and is significantly broadened due to the orbital motion of gas in the accretion disc. The broadening of the Balmer edge feature is therefore related to the quasar's inclination. This work proves the concept of determining quasar's inclination from the Balmer edge feature in their continuum spectra.