In this work the development of the simulation code Astro-GRIPS, the General Relativistic Implicit Parallel Solver, is performed, which solves the three-dimensional axi-symmetric general relativistic hydrodynamic Euler or Navier-Stokes equations under the assumption of a fixed background metric of a Schwarzschild or Kerr black hole using time-implicit methods. It is an almost total re-write of an old spaghetti-code like serial Fortran 77 simulation program. By modernization and optimization it is now a modern, well structured, user-friendly, flexible and extensible simulation program written in Fortran 90/95. The finite volume discretization ensures conservation and the defect-correction iteration strategy is used to resolve the non-linearities of the equations. One can use a variety of solution procedures that range from purely explicit up to fully implicit schemes with up to third order spatial and second order temporal accuracy. The large sparse linear equation systems used for the implicit methods can be solved by the Black-White Line-Gauß-Seidel relaxation method (BW-LGS), the Approximate Factorization Method (AFM) or by Krylov Subspace Iterative methods like GMRES. The optimal solution method and the coupling of equations is problem-dependent. Optimizations in the matrix construction, the MPI-Parallelization for distributed memory machines and several Newtonian and relativistic tests were conducted successfully.
The H.E.S.S. experiment, an array of four Imaging Cherenkov Telescopes, widened the horizon of Very High Energy (VHE) astronomy. Its unprecedented sensitivity is well suited for the study of new classes of expected VHE emitters, such as passive galactic nuclei that are the main focus of the work presented in this thesis. Acceleration of particles up to Ultra High Energies is expected in the magnetosphere of supermassive black holes (SMBH). The radiation losses of these accelerated particles are expected to reach the VHE regime in which H.E.S.S. operates. Predicted fluxes exceed the sensitivity of the array. However, strong photon fields in the surrounding of the acceleration region might absorb the produced radiation. Therefore observations focus on those galactic nuclei that are underluminous at lower photon energies. This work presents data collected by the H.E.S.S. telescopes on the test candidate NGC 1399 and their interpretation. While no detection has been achieved, important constraints can be derived from the obtained upper limits on the maximum energy attainable by the accelerated particles and on the magnetic field strength in the acceleration region. A limit on the magnetic field of B<74 Gauss is given. The limit is model dependent and a scaling of the result with the assumptions is given. This is the tightest empirical constraint to date. Because of the lack of signal from the test candidate, a stacking analysis has been performed on similar sources in three cluster fields. A search for signal from classes of active galactic nuclei has also been made in the same three fields. None of the analyzed samples revealed a significant signal. Also presented are the expectations for the next generation of Cherenkov Telescopes and an outlook on the relativistic effects expected on the VHE emission close to SMBH.
Der Ursprung der molekularen Infrarotemission im Zusammenhang mit protostellaren Jets der Klasse 0 und Klasse 1 (z.B. HH211 und HH46/47) ist nicht vollständig verstanden. Ein Modell, das diese Phänomene erfolgreich beschreibt ist das ”jet-driven outflow“ Modell. Dieses erklärt die Abstrahlung durch einen kollimierten Jet, der mit hoher Geschwindigkeit auf die molekulare Materie in der Umgebung trifft und diese in Schocks anregt und mit sich reit. Obgleich dieses Szenarium sehr erfolgreich die Dynamik und Morphologie des Ausflusses beschreibt, ist weiterhin unklar, ob Schocks des Typ J oder C diese Emission verursachen. Die physikalische Beschaffenheit des Gases, namentlich der Ionisierungsgrad und das Magnetfeld, sind wesentliche Parameter, die die genaue Art des Schock bestimmen. Da jedoch die direkte Umgebung von Klasse 0 Objekten aus dichtem Gas hoher Extinktionsrate innerhalb eines molekularen Kerns besteht, ist die direkte Beobachtung dieser Daten unmglich. Daher spielt die numerische Modellierung eine wichtige Rolle bei der Erforschung der beobachteten Ausflüsse. Wir haben ein Modul für den astrophysikalischen Simulationscode PLUTO entwickelt und getestet, das die molekulare Nichtgleichgewichtschemie und Khlung in einem Jet während seiner Interaktion mit dem molekularen Gas des umgebenden protostellaren Kerns simuliert. Unter Verwendung von großskaligen magnetohydrodynamischen Simulationen auf adaptivem Gitter finden wir bedeutende Infrarotemissionen von molekularem Gas das in J-Schocks angeregt wurde. Unsere Ergebnisse zeigen, dass diese Abstrahlung sowohl durch direkte Schocks (”prompt entrainment“) oder durch Abtragung und Abdampfung des Umgebungsgases verursacht werden kann. Die Eigenschaften der Emission sind stark von den absoluten und relativen Dichten der Jetmaterie und des Umgebungsgases und von dem Vorhandensein eines moderaten Magnetfeldes (in der Größenordnung 30 micro Gauss - 120 micro Gauss) um protostellaren Kern abhängig. Beim Vergleich mit Beobachtungen zeigt sich, das die berechnete Abstrahlungsintensität von der selben Größenordung wie die beobachtete ist. Wir zeigen wie die Emission verschiedener Quellen am Himmel von den hier untersuchten Parametern abhängt.
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 flow is assumed to cause the observational signatures of quiescent galaxies, 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 magnetorotational instability fed by the shearing of the magnetic field in the differential rotation of the disc. Direct observation of these objects is difficult 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 flows. 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 field, we found the crucial parameters that determine accretion and ejection. The results show that the initial geometry of the magnetic field is of almost no relevance for the long term behaviour. However, if the magnetic field strength is to small, no fast outflow 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.
The remarkable stability of astrophysical jets is not yet fully understood and requires further investigation. In order to study the effects of an antiparallel magnetic field topology on the linear stage and nonlinear evolution of the Kelvin Helmholtz (KH) instability, we performed direct numerical simulations to solve the ideal magnetohydrodynamic equations in a variety of initial configurations. Single shear layers presented growth rates of the instability higher than in the uniform (parallel) case, and a typical oscillation wave vector Ka 0.4. Vortical motions were observed for Alfv´en Mach numbers Ma > 2. The presence of tearing type magnetic islands, driven by the KH instability, reduced the magnetic field enhancement around the perimeter of the KH vortices proper of the KH instability and, subsequently, reduced the value of the magnetic saturation energy as compared to the uniform field case. The extended domain simulations showed an inverse cascade to bigger scales, more turbulent than in the uniform case. The lower magnetic amplification, due to the islands, moved the threshold for three-dimensional (3D) reorganization to a laminar flow from Ma . 50 (uniform) to Ma . 20 (antiparallel). Two-dimensional (2D) spatial slab-jet simulations showed episodic disruption and revival of the flow due to a magnetic field amplification process, previously believed to be present only in subsonic flows. This result, retrieved also in 3D simulations, is the same for uniform and antiparallel magnetic fields.
Diese Arbeit behandelt kompakte Sterne in der Branenwelt. Die Tolman- Oppenheimer-Volkoff-Gleichungen der stellaren Struktur werden für einen Stern auf der Brane hergeleitet. Um dieses Gleichungssystem zu schließen, wird angenommen, dass die Komponenten des projizierten Weyl-Tensors eine Art Zustandsgleichung P = wU erfüllen, was eine Relation zwischen dem isotropen und anisotropen Druck des effektiven Weyl-Fluids darstellt. Schließlich werden die Branen-TOV-Gleichungen für Neutronensterne und Weiße Zwerge numerisch gelöst, wobei insbesondere der inhomogene Dich- teverlauf im Sterninneren duch Verwendung realistischer Zustandsglei- chungen der Sternmaterie berücksichtigt wird. Aus dem Vergleich mit Be- obachtungsdaten ergeben sich Einschränkungen an die Parameter lambda (Bra- nenspannung) und w des Modells.