Polarization of Quasars: Electron Scattering in the Broad Absorption Line Region

Abstract

It is widely accepted that the broad absorption line region (BALR) exists in most (if not all) quasars with a small covering factor. Recent works showed that the BALR is optically thick to soft and even medium energy X-rays, with a typical hydrogen column density of a few 10${}^{23}$ to $>$ 10${}^{24}$ cm${}^{-2}$. The electron scattering in the thick absorber might contribute significantly to the observed continuum polarization for both BAL QSOs and non-BAL QSOs. In this paper, we present a detailed study of the electron scattering in the BALR by assuming an equatorial and axisymmetric outflow model. Monte-Carlo simulations are performed to correct the effect of the radiative transfer. Assuming an average covering factor of 0.2 of the BALR, which is consistent with observations, we find the electron scattering in the BALR with a column density of $\sim$ 4 $\times$ 10${}^{23}$ cm${}^{-2}$ can successfully produce the observed average continuum polarization for both BAL QSOs and non-BAL QSOs. The observed distribution of the continuum polarization of radio quiet quasars (for both BAL QSOs and non-BAL QSOs) is used to constrain the dispersal distribution of the BALR. We find that, to match the observations, the maximum continuum polarization produced by the BALR (while viewed edge-on) peaks at $P$ = 0.34%, which is much smaller than the average continuum polarization of BAL QSOs ($P$ = 0.93%). The discrepancy can be explained by a selection bias that the BAL with larger covering factor, and thus producing larger continuum polarization, is more likely to be detected. A larger sample of radio quiet quasars with accurate measurement of the continuum polarization will give better constraints to the distribution of the BALR properties.

17 pages,9 figures, accepted by APJ

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Cited by in corpus (8)

• Low-z MgII Broad Absorption-Line Quasars from the Sloan Digital Sky Survey
• The Correlation between X-ray and UV Properties of BAL QSOs
• The Compact Structure of Radio-Loud Broad Absorption Line Quasars
• Polarization microlensing in the quadruply imaged broad absorption line quasar H1413+117
• Polarization of Quasars: Resonant Line Scattering in the Broad Absorption Line Region
• Radiation-pressure Waves and Multiphase Quasar Outflows
• On the scattering enhancement of Nv1240 emission line of quasi-stellar objects
• Resolving the Structure at the Heart of BAL Quasars Through Microlensing Induced Polarisation Variability