Radio variability and interstellar scintillation of blazars
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(Front matter)
(Chapter 1)
(Chapter 2)
(Chapter 3)
(Chapter 4)
Date
2003
Authors
Bignall, Hayley Emma
Editors
Advisors
Clay, Roger William
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Thesis
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Abstract
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.
School/Discipline
Physics
Dissertation Note
Thesis (Ph.D.)--Physics, 2003.
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