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On the numerical simulation of advection dominated accretion flows [Elektronische Ressource] / put forward by Steffen Brinkmann

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109 pages
Dissertationsubmitted to theCombined Faculties for the Natural Sciences and for Mathematicsof the Ruperto-Carola University of Heidelberg, Germanyfor the degree ofDoctor of Natural SciencesPut forward byDipl.Phys. Ste en Brinkmannborn in Weseloral examination: 7.7.2009On the numerical simulation ofadvection dominated accretion owsReferees:Prof. Dr. Max CamenzindPriv.Doz. Dr. Hubert KlahrSte en Brinkmann: On the numerical simulation of advection dominated accre-tion ows, Dissertation submitted to the Combined Faculties for the Natural Sci-ences and for Mathematics of the Ruperto-Carola University of Heidelberg, Germanyfor the degree of Doctor of Natural Sciences,c 2009Abstract / Zusammenfassung:On the numerical simulation of advection dominated accretion owsIn accreting systems of low accretion rate (< 1% Eddington rate) the accretion isdominated by advection rather than by radiation or thermal convection. This typeof accretion ow is assumed to cause the observational signatures of quiescent galax-ies, X-ray binaries in the low-hard state and other compact objects. The motor thatmaintains the transport of angular momentum and accretion is the magnetorota-tional instability fed by the shearing of the magnetic eld in the di erential rotationof the disc. Direct observation of these objects is di cult due to their small size andbecause they are often hidden behind accreting matter.
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Dissertation
submitted to the
Combined Faculties for the Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
Put forward by
Dipl.Phys. Ste en Brinkmann
born in Wesel
oral examination: 7.7.2009On the numerical simulation of
advection dominated accretion ows
Referees:
Prof. Dr. Max Camenzind
Priv.Doz. Dr. Hubert KlahrSte en Brinkmann: On the numerical simulation of advection dominated accre-
tion ows, Dissertation submitted to the Combined Faculties for the Natural Sci-
ences and for Mathematics of the Ruperto-Carola University of Heidelberg, Germany
for the degree of Doctor of Natural Sciences,c 2009Abstract / Zusammenfassung:
On the numerical simulation of advection dominated accretion ows
In accreting systems of low accretion rate (< 1% Eddington rate) the accretion is
dominated by advection rather than by radiation or thermal convection. This type
of accretion ow is assumed to cause the observational signatures of quiescent galax-
ies, X-ray binaries in the low-hard state and other compact objects. The motor that
maintains the transport of angular momentum and accretion is the magnetorota-
tional instability fed by the shearing of the magnetic eld in the di erential rotation
of the disc. Direct observation of these objects is di cult due to their small size and
because they are often hidden behind accreting matter. Therefore simulating the
equations of ideal magnetohydrodynamics is necessary to investigate the structure
and dynamics of these accretion ows.
We carried out direct numerical simulations of an accreting torus around a black
hole. By performing tests varying the initial magnetisation, the size and position of
the torus and the geometry of the magnetic eld, we found the crucial parameters
that determine accretion and ejection. The results show that the initial geometry of
the magnetic eld is of almost no relevance for the long term behaviour. However,
if the magnetic eld strength is to small, no fast out ow arises. Also the presence
of radiative cooling will inhibit powerful ejections. Moreover we report retrograde
rotation of the jet with respect to the disc rotation.
Uber die numerische Simulation advektionsdominierter Akkretionsusse
In akkretierenden Systemen niedriger Akkretionsrate (< 1% der Eddington-Rate)
wird der Akkretions uss durch Advektion dominiert und nicht durch Wechsel-
wirkung mit Strahlung oder thermischer Konvektion. Dieser Akkretionstyp wird
gemeinhin fur die beobachteten Eigenschaften von inaktiven Galaxien, Rontgen-
Doppelsternen im Low-Hard-State\ und anderen kompakten Objekten verant-
"
wortlich gemacht. Der Mechanismus, der den Drehimpulstransport und damit die
Akkretion aufrechterhalt, ist die Magnetorotationsinstabilitat. Diese wird durch die
Scherung des Magnetfeldes in der di erenziell rotierenden Scheibe erzeugt. Die di-
rekte Beobachtung dieser Objekte ist schwierig aufgrund ihrer geringen Gro e und
weil sie hau g hinter dem akkretierenden Material verborgen sind. Daher ist es
notwendig, magnetohydrodynamische Simulationen dieser Objekte zu erstellen, um
ihre Dynamik und Struktur zu erforschen.
Wir haben direkte numerische Simulationen eines Akkretionstorus durchgefuhrt, in
denen wir verschiedene Anfangszustande getestet haben. Durch Variation der Po-
sition und Gro e des Torus, seiner Magnetisierung und der Geometrie des Magnet-
feldes, konnten wir die wesentlichen Parameter fur die globale Struktur ausmachen.
Es zeigt sich, dass die Geometrie des anfan glichen Magnetfeldes irrelevant fur die
Langzeitentwicklung der Simulation ist. Wenn das Magnetfeld jedoch zu schwach ist,
wird kein schneller, Jet-artiger Materieuss gebildet. Kuhlung durch Abstrahlung
verhindert ebenfalls starke Aus usse. Au erdem fanden wir retrograde Rotation
des Jets bezuglich der Scheibenrotation.God is always there. But you got to do the footwork.
Mike ‘Flea’ Balzary

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