Optical polarization of quasars and the Balmer edge feature revealed by ultraviolet and polarized visible to near-infrared emissions

Authors

  • Renyu Hu,

    1. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
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  • Shuang-Nan Zhang

    Corresponding author
    1. Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China
    2. Space Science Division, National Astronomical Observatories of China, Chinese Academy of Sciences, Beijing, China
    • Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
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E-mail: zhangsn@ihep.ac.cn

ABSTRACT

Polarized emission from a quasar is produced by wavelength-independent electron scattering that surrounds its accretion disc. Thus, contamination from the host galaxy is avoided and the intrinsic emission spectrum of its accretion disc is revealed. Ultraviolet (UV) emission from a quasar is normally free of contamination from its host galaxy. Therefore, the polarization fraction of the quasar's disc emission can be determined by comparing the total UV emission with the polarized visible to near-infrared (NIR) emission. The resulting continuum spectrum from UV to infrared can reveal the theoretically expected Balmer edge absorption feature. We fit the polarized spectra in the visible and NIR bands, together with the total UV spectra of two type 1 quasars (3C 95 and 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 that results from corotational neutral gas on a narrow annulus of the accretion disc. We find that the extended thin accretion disc model provides an adequate description of 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 constrain well the true polarization fraction of the disc emission, which allows the Balmer edge feature to be completely revealed from polarized visible to UV continua. The Balmer edge feature is prominent in the spectra of both quasars, and it is significantly broadened because of the orbital motion of the gas in the accretion disc. Therefore, the broadening of the Balmer edge feature is related to the quasar's inclination. In this paper, we prove the concept of determining a quasar's inclination from the Balmer edge feature in its continuum spectra.

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