Multi-frequency synchrotron self-compton models for the brightness temperature problem in compact extra-galactic radio sources [Elektronische Ressource] / presented by Olivia Tsang

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Dissertation submitted to theCombined Faculties for the Natural Sciences and Mathematicsof the Ruperto-Carola University of Heidelberg, Germanyfor the degree ofDoctor of Natural Sciencespresented byOlivia Tsang, MSci Physicsborn in Hong KongOral examination: 25 July 2007Multi-frequency Synchrotron Self-Compton Modelsfor the Brightness Temperature Problemin Compact Extra-galactic Radio SourcesReferees:Prof. Dr. John G. KirkProf. Dr. Stefan WagnerAbstractFlux variations in quasars and BL Lac objects over a time scale of a day orless suggest an extremely high brightness temperature in these sources, which can-not be explained by conventional synchrotron theory. This work addresses the issueof extreme brightness temperature by applying synchrotron theory to unconventionalelectron distributions.We consider a scenario in which relativistic electrons are continuously injectedinto the emission region. In the first approximation, we assume the electrons aremonoenergetic for simplicity. This approximation is insufficient when modelling thespectrum of S5 0716+714, we therefore modified the electron injection spectrum to onewhich is a double power law in energy. This retains the low radio frequency spectralcharacteristics of monoenergetic electrons, which extends to higher frequencies as apower law. To complete the study of the intrinsic properties of synchrotron emissionfrom monoenergetic electrons, we also examine their circular polarisation.

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Published 01 January 2007
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Dissertation submitted to the
Combined Faculties for the Natural Sciences and Mathematics
of the Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
presented by
Olivia Tsang, MSci Physics
born in Hong Kong
Oral examination: 25 July 2007Multi-frequency Synchrotron Self-Compton Models
for the Brightness Temperature Problem
in Compact Extra-galactic Radio Sources
Referees:
Prof. Dr. John G. Kirk
Prof. Dr. Stefan WagnerAbstract
Flux variations in quasars and BL Lac objects over a time scale of a day or
less suggest an extremely high brightness temperature in these sources, which can-
not be explained by conventional synchrotron theory. This work addresses the issue
of extreme brightness temperature by applying synchrotron theory to unconventional
electron distributions.
We consider a scenario in which relativistic electrons are continuously injected
into the emission region. In the first approximation, we assume the electrons are
monoenergetic for simplicity. This approximation is insufficient when modelling the
spectrum of S5 0716+714, we therefore modified the electron injection spectrum to one
which is a double power law in energy. This retains the low radio frequency spectral
characteristics of monoenergetic electrons, which extends to higher frequencies as a
power law. To complete the study of the intrinsic properties of synchrotron emission
from monoenergetic electrons, we also examine their circular polarisation.
We find that (1) electron distribution with low energy cut-off is able to generate
high brightness temperature, and (2) the flat synchrotron spectrum produced by such
distribution is in good agreement with that of the observed, and (3) in contrast to a
power-law distribution, circular polarisation of synchrotron emission from monoener-
getic electrons does not change sign.
Zusammenfassung
Die Beobachtung von Helligkeitsver¨anderungen in Quasaren und BL Lac Ob-
jekten auf einer Zeitskala von Tagen oder weniger, legt eine extrem hohe Helligkeit-
stemperatur in diesen Quellen nahe, die sich nicht ohne weiteres aus den bisherigen
Standard-Synchrotrontheorie-Ans¨atzenerklare¨ n l¨asst. Diese Arbeit untersucht daher
das Problem extremer Helligkeitstemperaturen im Zusammenhang der Synchrotron-
theorie fu¨r unkonventionelle Elektronverteilungen.
WirbetrachtendazueinModell,beidemrelativistischeElektronenkontinuierlich
in das Emissionsgebiet injeziert werden. In einer ersten N¨aherung nehmen wir der Ein-
fachheit halber an, dass die Elektronen mononenergetisch sind. Diese N¨aherung reicht
allerdings noch nicht aus, um z.B. das Spektrum des BL Lac Objektes S5 0716+714
zu modellieren. Wir fu¨hren daher eine modifizierte Elektronverteilung ein, welche
einem doppeltem Potenzgesetz in der Energie folgt. Diese ist so gew¨ahlt, dass sie
die Niederfrequenz-Radio-Spektralcharakteristik monoenegetischer Elektronen erh¨alt
und zu h¨oheren Frequenzen hin einem Potenzgesetz folgt. Zur vollst¨andigen Analyse
der intrinsischen Synchrotronemission monoenergetischer Elektronen untersuchen wir
außerdem die zirkularen Polarisationseigenschaften.
UnsereArbeitzeigt,dassentprechendeElektronverteilungenmiteinerNiederenergie-
Grenze durchaus in der Lage sind, (1) das Problem der hohen Helligkeitstemperaturen
zu l¨osen und (2) den beobachteten, flachen Spektralverlauf erfolgreich zu erkl¨aren, und
dass (3) die zirkulare Polarisation der Synchrotronstrahlung monoenegetischer Elek-
tronen, im Gegensatz zu Potenzgesetz-Verteilungen, das Vorzeichen nicht wechselt.To Nick, my parents and WicketAcknowledgements
I would like to take this opportunity to thank...
...John Kirk,myPhDadvisor,forhispatientguidanceandfornevercomplainingabout
the number of drafts I have asked him to read.
...Stefan Wagner for agreeing to be the second reader of my thesis and suggestions
given to my work.
...Klaus Meisenheimer and Werner Hoffman for agreeing to be the examiners of my
viva voce.
...Nick, my husband, for his support and entertainment over the years.
...My parents for telling me not to be so lazy for the past twenty years.
...My parents in law for their support.
...Andrea Ma and Mario Kallis for their distractions, a sign of true friendship.
...Richard Tuffs for proof reading parts of my thesis.
...Frank Rieger for ”help” with the German abstract.
...Jerome Petri for help with Mathematica.
...DimitriosEmmanoulopoulosforbeinganotherPhD-student-who-cannot-speak-German-
very-well.
...Luisa Ostorero for information and advices.
...Dmitry Khangulyan for telling me that thesis is not so important.
...Andrew Taylor for agreeing to proof read my introduction but never got back to me.
...The Astrophysics groupattheMPIKforfrequentlyaskingwhenIwillfinishmyPhD.
...Wicket the cat for being a constant amusement.
and finally all the people that I should thank but have not been included here due to
space restriction. THANK YOU!