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The Difference between BL Lacs and QSOs

from I - Evidence and Implications of Anisotropy in AGN

Published online by Cambridge University Press:  04 August 2010

P. A. Hughes
Affiliation:
Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA
M. F. Aller
Affiliation:
Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA
H. D. Aller
Affiliation:
Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA
D. C. Gabuzda
Affiliation:
Department of Physics and Astronomy, University of Calgary, Calgary Alberta T2N 1N4 CANADA
Andrew Robinson
Affiliation:
University of Cambridge
Roberto Juan Terlevich
Affiliation:
Royal Greenwich Observatory, Cambridge
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Summary

Abstract

We present evidence that BL Lac objects are a distinct class, rather than QSOs viewed close to, or within, the ‘critical cone’ of their collimated flow: both statistical analysis of the Stokes parameters and VLB polarimetry imply that the magnetic field structures in BL Lacs and QSOs are intrinsically different.

The low degree of polarization in the quiescent state of radio variable AGN, and the occurrence of occasional “rotation” events, have provided evidence for the presence of a highly tangled magnetic field. We would anticipate the Stokes parameters (Q, U) versus time plots of BL Lac objects and QSOs to show systematic differences if the relative strengths of tangled and ordered field components differ between classes. Fig. 1 illustrates typical Q-U plots for each class, derived from single dish observations, showing that the dispersion is large compared to the offset from zero for the BL Lac (interpreted as due to the presence of a weak mean field, so that evolution is dominated by the random walk of points in the Q-U plane as ‘new’ magnetic cells are advected into the window of observation), but small compared to the offset for the QSO (interpreted as due to a stronger mean field: the random walk is now about a point displaced from the origin by virtue of this stronger, axial field). A statistical analysis of 51 sources confirms this trend.

Type
Chapter
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The Nature of Compact Objects in Active Galactic Nuclei
Proceedings of the 33rd Herstmonceux Conference, held in Cambridge, July 6-22, 1992
, pp. 94 - 95
Publisher: Cambridge University Press
Print publication year: 1994

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  • The Difference between BL Lacs and QSOs
    • By P. A. Hughes, Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA, M. F. Aller, Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA, H. D. Aller, Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA, D. C. Gabuzda, Department of Physics and Astronomy, University of Calgary, Calgary Alberta T2N 1N4 CANADA
  • Edited by Andrew Robinson, University of Cambridge, Roberto Juan Terlevich, Royal Greenwich Observatory, Cambridge
  • Book: The Nature of Compact Objects in Active Galactic Nuclei
  • Online publication: 04 August 2010
  • Chapter DOI: https://doi.org/10.1017/CBO9780511564765.023
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  • The Difference between BL Lacs and QSOs
    • By P. A. Hughes, Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA, M. F. Aller, Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA, H. D. Aller, Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA, D. C. Gabuzda, Department of Physics and Astronomy, University of Calgary, Calgary Alberta T2N 1N4 CANADA
  • Edited by Andrew Robinson, University of Cambridge, Roberto Juan Terlevich, Royal Greenwich Observatory, Cambridge
  • Book: The Nature of Compact Objects in Active Galactic Nuclei
  • Online publication: 04 August 2010
  • Chapter DOI: https://doi.org/10.1017/CBO9780511564765.023
Available formats
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Save book to Google Drive

To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.

  • The Difference between BL Lacs and QSOs
    • By P. A. Hughes, Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA, M. F. Aller, Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA, H. D. Aller, Astronomy Department, University of Michigan, Ann Arbor MI 48109-1090 USA, D. C. Gabuzda, Department of Physics and Astronomy, University of Calgary, Calgary Alberta T2N 1N4 CANADA
  • Edited by Andrew Robinson, University of Cambridge, Roberto Juan Terlevich, Royal Greenwich Observatory, Cambridge
  • Book: The Nature of Compact Objects in Active Galactic Nuclei
  • Online publication: 04 August 2010
  • Chapter DOI: https://doi.org/10.1017/CBO9780511564765.023
Available formats
×