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Anisotropic induced Compton scattering - Constraints on models of active galactic nuclei

Coppi, P. and Blandford, R. D. and Rees, M. J. (1993) Anisotropic induced Compton scattering - Constraints on models of active galactic nuclei. Monthly Notices of the Royal Astronomical Society, 262 (3). pp. 603-618. ISSN 0035-8711.

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Induced Compton scattering can cause strong spectral distortion when the Thomson scattering optical depth τT exceeds 5 × 10^9 K/T, where T is the brightness temperature. Observations of intraday variability in several compact radio sources and direct brightness temperature measurements of ~ 1–4 × 10^(12) K from orbiting VLBI prompt a re-examination of this process. We seek observable signatures of induced scattering, using a model for non-linear radiative transfer on a lattice. Induced Compton scattering can significantly increase the brightness temperature at low frequencies and may lead to the formation of steep gradients in the spectrum, and angular discontinuities in the observed intensity. An ultracompact high-brightness core surrounded by a tenuous electron-scattering shell should thus show a faint halo whose spectrum peaks at a lower frequency than the core. Thicker shells can be strongly limb-brightened; frequency-dependent superluminal expansion is also possible. The strongest signatures of induced Compton scattering are strong spectral variation in the source structure and large, frequency-dependent, linear polarization. Under some conditions, stimulated Raman scattering off collective plasma waves, which has similar observational characteristics, may dominate over induced Compton scattering off individual electrons and screened ions. As yet, there is no compelling evidence that induced Compton scattering has been observed in compact radio sources. This implies a (model-dependent) upper limit on the Thomson optical depth of plasma in the vicinity of the source. Future VLBI observations, with greater dynamic range, may provide valuable tracers of dense ionized gas in active galactic nuclei. Infrared observations of blazars should be scrutinized for evidence of induced scattering.

Item Type:Article
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Blandford, R. D.0000-0002-1854-5506
Additional Information:© 1993 Royal Astronomical Society. Provided by the NASA Astrophysics Data System. Accepted 1992 November 5. Received 1992 November 4; in original form 1992 September 24. We thank Sterl Phinney, Tony Readhead and George Rybicki for valuable suggestions and critical comments on an earlier draft of this paper. RB thanks the Institute of Astronomy for hospitality during 1987 July-August when this work was begun and the Harvard-Smithsonian Center for Astrophysics for further support. NSF grant AST-89-17765 and NASA grants NAGW-830, NAGW-1284, NAGW-1301, NAGW-1636 and NAGW-2816 are also gratefully acknowledged.
Funding AgencyGrant Number
NSFAST 89-17765
Subject Keywords:radiation mechanisms: miscellaneous, galaxies: active, galaxies: nuclei
Issue or Number:3
Record Number:CaltechAUTHORS:20190523-125439272
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Official Citation:P. Coppi, R. D. Blandford, M. J. Rees, Anisotropic induced Compton scattering – Constraints on models of active galactic nuclei, Monthly Notices of the Royal Astronomical Society, Volume 262, Issue 3, 1 June 1993, Pages 603–618,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95754
Deposited By: George Porter
Deposited On:24 May 2019 17:05
Last Modified:03 Oct 2019 21:16

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