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Numerical simulations of blazar jets and their non-thermal radiation [Elektronische Ressource] / angefertigt von Petar Mimica

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Numerical Simulations of Blazar Jets andtheir Non-thermal RadiationDissertationder Fakult at fur PhysikderLudwig-Maximilians-Universitat Munc henangefertigt vonPetar Mimicaaus MimiceMunc hen, den September 22, 20041. Gutachter: Prof. Dr. Gerhard B orner2.hter: Prof. Dr. Harald LeschTag der mundlic hen Prufung: 22. November 200423Tece i tece, tece jedan slap;Sto u njem znaci moja mala kap?Gle, jedna duga u vodi se stvara,I sja i drsce u hiljadu sara.Taj san u slapu da bi mogo sjati,I moja kaplja pomaze ga tkati.\Slap" - Dobrisa Cesaric4ContentsAbstract 71 Introduction 111.1 Active galactic nuclei . . . . . . . . . . . . . . . . . . . . . . 111.2 Observations of AGNs . . . . . . . . . . . . . . . . . . . . . 131.2.1 AGN morphology . . . . . . . . . . . . . . . . . . . . 141.2.2 AGN classi cation . . . . . . . . . . . . . . . . . . . 151.2.3 Parsec scale jets . . . . . . . . . . . . . . . . . . . . . 181.3 BL Lac objects . . . . . . . . . . . . . . . . . . . . . . . . . 181.3.1 Blazar spectra . . . . . . . . . . . . . . . . . . . . . . 201.3.2 Variability . . . . . . . . . . . . . . . . . . . . . . . . 221.3.3 Blazar X-ray light curves . . . . . . . . . . . . . . . . 231.4 Theoretical models . . . . . . . . . . . . . . . . . . . . . . . 251.4.1 Apparent superluminal motion . . . . . . . . . . . . . 251.4.2 AGN uni cation . . . . . . . . . . . . . . . . . . . . 261.4.3 Internal shock model . . . . . . . . . . . . . . . . . .

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Published 01 January 2004
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Numerical Simulations of Blazar Jets and
their Non-thermal Radiation
Dissertation
der Fakult at fur Physik
der
Ludwig-Maximilians-Universitat Munc hen
angefertigt von
Petar Mimica
aus Mimice
Munc hen, den September 22, 2004
1. Gutachter: Prof. Dr. Gerhard B orner
2.hter: Prof. Dr. Harald Lesch
Tag der mundlic hen Prufung: 22. November 200423
Tece i tece, tece jedan slap;
Sto u njem znaci moja mala kap?
Gle, jedna duga u vodi se stvara,
I sja i drsce u hiljadu sara.
Taj san u slapu da bi mogo sjati,
I moja kaplja pomaze ga tkati.
\Slap" - Dobrisa Cesaric4Contents
Abstract 7
1 Introduction 11
1.1 Active galactic nuclei . . . . . . . . . . . . . . . . . . . . . . 11
1.2 Observations of AGNs . . . . . . . . . . . . . . . . . . . . . 13
1.2.1 AGN morphology . . . . . . . . . . . . . . . . . . . . 14
1.2.2 AGN classi cation . . . . . . . . . . . . . . . . . . . 15
1.2.3 Parsec scale jets . . . . . . . . . . . . . . . . . . . . . 18
1.3 BL Lac objects . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.3.1 Blazar spectra . . . . . . . . . . . . . . . . . . . . . . 20
1.3.2 Variability . . . . . . . . . . . . . . . . . . . . . . . . 22
1.3.3 Blazar X-ray light curves . . . . . . . . . . . . . . . . 23
1.4 Theoretical models . . . . . . . . . . . . . . . . . . . . . . . 25
1.4.1 Apparent superluminal motion . . . . . . . . . . . . . 25
1.4.2 AGN uni cation . . . . . . . . . . . . . . . . . . . . 26
1.4.3 Internal shock model . . . . . . . . . . . . . . . . . . 27
1.5 Existing numerical methods used to produce synthetic light
curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
1.6 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2 Non-thermal radiation 35
2.1 Synchrotron . . . . . . . . . . . . . . . . . . . . . 35
2.1.1 Radiation emitted by a single charged particle . . . . 35
2.1.2 by a distribution of particles . . . 41
2.1.3 Radiation from a power law of particles . 45
2.2 Inverse-Compton scattering . . . . . . . . . . . . . . . . . . 46
2.2.1 Scattering of the monochromatic radiation . . . . . . 46
2.2.2 of radiation whose intensity per unit fre-
quency is a power law . . . . . . . . . . . . . . . . . 52
3 Non-thermal particles 57
3.1 Acceleration of particles at the shock fronts . . . . . . . . . 57
3.1.1 Non-relativistic shocks . . . . . . . . . . . . . . . . . 57
3.1.2 Relativistic shocks . . . . . . . . . . . . . . . . . . . 59
3.2 Evolution of non-thermal particles . . . . . . . . . . . . . . . 62
3.2.1 Kinetic equation . . . . . . . . . . . . . . . . . . . . 62
56 CONTENTS
4 Numerical method 67
4.1 Numerical RHD . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.1.1 Applicability of the ideal RHD approximation . . . . 67
4.1.2 Equations of ideal RHD . . . . . . . . . . . . . . . . 68
4.1.3 RHD numerical scheme . . . . . . . . . . . . . . . . . 69
4.2 Treatment of the non-thermal particles . . . . . . . . . . . . 71
4.2.1 Spatial transport: non-thermal particles as tracers . . 71
4.2.2 The kinetic equation solver . . . . . . . . . . . . . . . 71
4.2.3 The treatment of the source term . . . . . . . . . . . 73
4.3 Non-thermal radiation . . . . . . . . . . . . . . . . . . . . . 76
4.3.1 Treatment of the synchrotron radiation . . . . . . . . 76
4.3.2 Light curve computation . . . . . . . . . . . . . . . . 78
4.4 Method validation . . . . . . . . . . . . . . . . . . . . . . . 81
4.4.1 Tests of the kinetic-equation solver . . . . . . . . . . 81
4.4.2 Test of the synchrotron radiation code . . . . . . . . 86
5 Simulations of blazar ares 91
5.1 Introduction to blazar simulations . . . . . . . . . . . . . . . 91
5.1.1 The physical model . . . . . . . . . . . . . . . . . . . 91
5.1.2 Numerical details of the simulations . . . . . . . . . . 92
5.2 Two dimensional simulations . . . . . . . . . . . . . . . . . . 95
5.2.1 Hydrodynamic evolution . . . . . . . . . . . . . . . . 95
5.2.2 S0 light curve . . . . . . . . . . . . . . . . . . . . . . 98
5.2.3 In uence of the density . . . . . . . . . . . . . . . . . 100
5.2.4 of the pressure . . . . . . . . . . . . . . . . 101
5.3 One dimensional simulations . . . . . . . . . . . . . . . . . . 102
5.3.1 Spacetime analysis of the light curve . . . . . . . . . 103
5.3.2 Light curves of one-dimensional models . . . . . . . . 105
5.3.3 Spacetime evolution of the emissivity . . . . . . . . . 111
5.3.4 Spectral ev . . . . . . . . . . . . . . . . . . . . 111
5.4 Analytic are model . . . . . . . . . . . . . . . . . . . . . . 116
5.4.1 Application to models G0E, G1E and G2E . . . . . . 127
5.5 Comparison with observations of blazar ares . . . . . . . . 130
5.6 Simulations of multiple collisions . . . . . . . . . . . . . . . 132
5.6.1 A model of an inhomogeneous blazar jet . . . . . . . 132
6 Summary and conclusions 137
6.1 Summary of the results . . . . . . . . . . . . . . . . . . . . . 137
6.2 Next steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
A The tting algorithm 141
List of tables 145
List of gures 145
Bibliography 148Abstract
In the past years relativistic (magneto)hydrodynamic simulations have been
used extensively to study the time-dependent hydrodynamic properties of
extra galactic jets. While these simulations have been very successful in
studying the formation, collimation, propagation and termination of rela-
tivistic jets, the models used to compute synthetic images from the hydro-
dynamic properties were relatively simple (but see, e.g., Jones et al. (1999)
for an example of a more sophisticated model). On the other hand, there
exist several theoretical models which assume a very simple hydrodynamic
evolution, but treat the non-thermal particles and their emitted radiation
with great detail.
It was the aim of this work to include a detailed treatment of the non-
thermal particles and their synchrotron radiation in high-resolution shock-
capturing relativistic hydrodynamic (RHD) simulations. To achieve this
goal we have developed a transport scheme for the non-thermal particles
by treating them as \tracer" uids in the RHD equations. Their temporal
evolution is calculated using an analytic kinetic equation solver, and their
synchrotron radiation is computed in a time-dependent manner taking into
account the relevant relativistic e ects, (e.g., light travel times to the ob-
server). The energy density of a dynamically negligible magnetic eld is
assumed to be a fraction of the energy density of the thermal uid. Two
models have been developed for the parameterization of the acceleration of
non-thermal particles at relativistic shocks: A type-E model where only the
strength of the shock in uences the number of accelerated particles and a
type-N model where the shock strength only in uences their energy distri-
bution.
We have demonstrated that our numerical method is able to capture the
essentials of the temporal and spatial evolution of the non-thermal particles
and the observed synchrotron radiation with a reasonable accuracy when
applied to subparsec scale relativistic jets.
Understanding the physical processes connected to the observed X-ray
blazar light curves has been the main object of research with our new nu-
merical tool. For the rst time, the hydrodynamic evolution and the syn-
chrotron radiation of a blazar jet was simulated consistently. We have sim-
ulated collisions of density inhomogeneities (shells) within a blazar jet. The
results have shown that the e ciency of the observed synchrotron radiation
varies with the relative velocities of the shells as well as with the amount of
initially available mass. The surrounding medium plays an important role,
because it heats up the shells prior to the collision, a fact which is neglected
78 ABSTRACT
in simpler models.
Assuming that the observed radiation results from the interaction of
shells within a blazar jet, we have developed an analytic model which enables
the determination of the unobservable parameters of the jets (i.e., length
and velocity of the shells) from the light curve. The parameters predicted
by the model have been compared to results of our simulations and we nd
that the agreement is surprisingly good, given the simplicity of the model.
In addition, several long-term simulations of collisions of many shells
have shown that a model of an intermittently working central engine seems
to produce light curves more similar the observed ones than a model in
which the central engine ejects a continuous out o w.ABSTRACT 9Astronomy treated its students kindly, providing many
tasks that were true, solid science and even might lead
to an important result. The universe was still so poorly
known that surprises lurked everywhere, especially when
one had a new instrument with greater seeing power,
or the ability to peer into the fresh region of the spec-
trum. The newer ’scopes were mostly distant hardware
operated by a corps of technicians. Astronomers them-
selves ruled these by long distance, asking for spots in
the night sky to be scrutinized, all over a Net connec-
tion. Nobody squinted through eyepieces anymore.
from "Eater" - Gregory Benford