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On the polarization of resonantly scattered emission lines -- III. Polarization of quasar broad emission lines and broad absorption line troughs

Lee, H.-W. and Blandford, R. D. (1997) On the polarization of resonantly scattered emission lines -- III. Polarization of quasar broad emission lines and broad absorption line troughs. Monthly Notices of the Royal Astronomical Society, 288 (1). pp. 19-42. ISSN 0035-8711. https://resolver.caltech.edu/CaltechAUTHORS:20190523-125438959

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Abstract

The contribution to the expected linear polarization of quasar broad emission and absorption lines from resonance scattering is computed using a Monte Carlo approach for specific, generic models. Attention is focused on the external illumination of the scattering region. The polarization of the reflected component from an externally illuminated slab with finite optical depth is first considered as a model of a single, dense, broad emission-line cloud. A polarization ≾ 40 per cent (≾10 per cent) is typically computed for J=0→1 (J=1/2ₒ3/2) transitions. Integration over a population of emission-line clouds typically reduces the observed polarization by a factor ∼(0.1–0.3)q_(BELR), where q_(BELR) is the covering factor for emission lines, while preserving the relative polarizations of different lines. Consequently, the low degree of polarization observed in the red wings of quasar emission lines limits the density of low column density emission-line clouds. Broad absorption lines are associated with outflowing gas of lower density. The polarization of both the transmitted and the reflected radiation is next computed for simple kinematic models of the outflow and the observed integrated polarization in the absorption-line troughs is found to be typically ∼ 10 per cent. An equatorial flow model gives a large degree of polarization (∼0.15) parallel to the symmetry axis in the absorption trough for the doublet transition J = 1/2→1/2, 3/2, and the polarized flux is found to extend to the red side. In contrast, we obtain a smaller degree of polarization ( ∼ 0.05) perpendicular to the jet axis from a bipolar flow model and the polarized flux is concentrated to the blue side of the line profile. It is predicted that the troughs of singlet J=0 →1 lines, such as C III λ 977, should exhibit larger degrees of polarization if this is due to resonance scattering. Polarization observations of quasar emission lines promise to be a powerful diagnostic of the kinematics of gas in the central pc of a quasar.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1093/mnras/288.1.19DOIArticle
http://adsabs.harvard.edu/abs/1997MNRAS.288...19LADSArticle
ORCID:
AuthorORCID
Blandford, R. D.0000-0002-1854-5506
Additional Information:© 1997 RAS. Accepted 1997 January 3. Received 1996 May 24; in original form 1995 September 25. We are indebted to several colleagues, notably Ski Antonucci, Nahum Arav, Marshall Cohen, Bob Goodrich, Ari Laor, Norm Murray, Patrick Ogle and Ray Weymann for invaluable discussions and helpful suggestions. Support under NSF grant AST 92-23370 and NASA grant NAGW 2372 is gratefully acknowledged.
Group:TAPIR
Funders:
Funding AgencyGrant Number
NSFAST 92-23370
NASANAGW 2372
Subject Keywords:polarization, galaxies: kinematics and dynamics, quasars: absorption lines, quasars: emission lines
Issue or Number:1
Record Number:CaltechAUTHORS:20190523-125438959
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20190523-125438959
Official Citation:H.-W. Lee, R. D. Blandford, On the polarization of resonantly scattered emission lines — III. Polarization of quasar broad emission lines and broad absorption line troughs, Monthly Notices of the Royal Astronomical Society, Volume 288, Issue 1, 11 June 1997, Pages 19–42, https://doi.org/10.1093/mnras/288.1.19
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95751
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:24 May 2019 16:51
Last Modified:03 Oct 2019 21:16

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