Blazar Times - No. 58 - October 2003
The Blazar Times
A Research Newsletter Dedicated to the BL Lac and Blazar Phenomena
No. 58 - October 2003 Editor: Travis A. Rector (


Thesis Abstracts 1

Journal Abstracts 2

Abstract Guidelines 3

Thesis Abstracts

Radio Variability and Interstellar Scintillation of Blazars

Hayley E. Bignall

Thesis work conducted at: Department of Physics, University of Adelaide, Australia

Current address: Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, the Netherlands


Ph.D dissertation directed by: Dr. Roger Clay (U. Adelaide), Dr. Tasso Tzioumis (ATNF)

Ph.D degree awarded: August 2003

This thesis presents several observational studies based on radio variability and interstellar scintillation of extragalactic flat-spectrum radio sources. Such sources are commonly called ``blazars'', a term used to describe the phenomenon observed when the jet of a radio-loud Active Galactic Nucleus (AGN) is directed towards the observer. These sources provide unique ``laboratories'' for studying the physics of relativistic jets.

Observations of selected samples of blazars, made with the Australia Telescope Compact Array (ATCA) and the Australia Telescope Long Baseline Array are presented here. Statistics for long-term (months-years) and short-term (intraday) variability in both total and linearly polarized flux density at several frequencies are presented. The sensitivity and flux density measurement accuracy of the ATCA make it particularly useful for observations of intraday variability (IDV). Resolving the question of what is the mechanism for radio IDV was of great importance at the time this thesis was being undertaken, since if intrinsic, IDV implies extremely high brightness temperatures, far in excess of the Inverse Compton limit for incoherent synchrotron radiation. Most source models are fundamentally based on the assumption that the radiation from radio to optical, and sometimes soft X-ray, energies is produced by the incoherent synchrotron mechanism, so any result which challenges this has serious implications.

There is now strong evidence that interstellar scintillation (ISS) is the principal cause of radio IDV, which substantially lowers the implied source brightness temperatures from those calculated assuming intrinsic variability. Some of the results presented in this thesis have made an important contribution to the ``paradigm shift'' from IDV to ISS, by showing unequivocally that the rapid IDV observed in PKS 1257-326 is a result of scintillation due to a nearby scattering ``screen'' in the ionised interstellar medium (ISM) of our Galaxy. This unusual source, serendipitously discovered during the course of my PhD, has also proved extremely valuable in showing that ISS can be used as a probe of microarcsecond-scale source structure and also of the local Galactic ISM. Such high angular resolution is not currently achievable even with space interferometer baselines.

Thesis-related Publications: ApJ, 585, 653

Journal Abstracts

Parsec Scale Properties of Markarian 501

M. Giroletti1,2, G. Giovannini1,2, L. Feretti1, W.D. Cotton3, P.G. Edwards4, L. Lara5,6, A.P. Marscher7, J.R. Mattox8, B.G. Piner9 and T. Venturi1

1 Istituto di Radioastronomia, via Gobetti 101, 40129, Bologna, Italy
2 Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
3 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
4 Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
5 Dpto. Fisica Teorica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
6 Instituto de Astrofisica de Andalucia (CSIC), Apdo. 3004, 18080 Granada, Spain
7 Institute for Astrophysical Research, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
8 Department of Physics and Astronomy, Francis Marion University, Florence, SC 29501-0547, USA
9 Department of Physics and Astronomy, Whittier College, 13406 East Philadelphia Street, Whittier, CA 90608, USA

We present the results of a high angular resolution study of the BL Lac object Markarian 501 in the radio band. We consider data taken at 14 different epochs, ranging between 1.6 GHz and 22 GHz in frequency, and including new Space VLBI observations obtained on 2001 March 5 and 6 at 1.6 and 5 GHz. We study the kinematics of the parsec-scale jet and estimate its bulk velocity and orientation with respect to the line of sight. Limb brightened structure in the jet is clearly visible in our data and we discuss its possible origin in terms of velocity gradients in the jet. Quasi-simultaneous multi-wavelength observations allow us to map the spectral index distribution and to compare it to the jet morphology. Finally, we estimate the physical parameters of the parsec-scale jet.

Accepted by ApJ

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Abstract Guidelines

Abstracts for ``The Blazar Times" are solicited for papers that have been recently accepted for publication by a refereed journal, and for recent Ph.D. theses. Please do not submit an abstract before it has been accepted, nor after it is published. Abstracts from papers which are not refereed (e.g., conference proceedings) are not accepted.

The subject matter should pertain directly to the BL Lac and/or blazar phenomenon in general. Both observational and theoretical abstracts are appropriate. Abstracts from papers dealing with other classes of AGN will generally not be included unless they explicitly discuss their relevance to the blazar phenomenon; however exceptions to this rule will be considered.

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On 17 Oct 2003, 15:55.