Welcome to my home page. You will find
information about my scientific interests, recent projects and a list
of my publications.
24.05.2011: We have our new paper on synchrotron radiation on the ArXiv.
My main research interests are connected to dynamical astrophysical systems and numerical simulations thereof.
Relativistic jets originate in the direct vicinity of the massive black holes residing in the centers of most galaxies. Either they are launched as an initially sonic wind of the accretion disk or they originate directly in the black holes ergosphere transforming rotational energy into high Lorentz factors via magneto hydrodynamic processes.
Here are two movies of RMHD simulations perfomed with the PLUTO code.
Here is a movie from a simulation of a jet-propagation showing the wiggling of the jet head.
Below is a simple synchotron raycasting through a jet formation simulation with peak Lorentz-factor of 4.5.
Once a star enters the tidal radius of the central black hole, it emmits an X-ray flare while part of its mass is being accreted to the hole. We have performed direct nbody simulations with the MPI-parallel code Nbody6++ on the various supercomputer sites that are part of the DEISA project. We implemented particle disruption to study the coupling between the dynamical and the relaxtational timescales (the classical losscone problem) by making no limiting assumptions on the stellar dynamics.
Our simulations confirm classical estimates of the losscone diffusion and predict a stellar disruption event in the Milkyway once every 10 000 years.
A central massive black hole imprints a clear signature onto the stellar system. The density profile approaches a Bahcall-Wolf cusp (slope -7/4) and the cluster expands. Here are two plots showing the cluster evolution and the disruption rate.
24.05.2011: We have our new paper on synchrotron radiation on the ArXiv.
My main research interests are connected to dynamical astrophysical systems and numerical simulations thereof.
Relativistic jet formation
In my PhD project i study the magneto hydrodynamic formation of relativistic jets around compact objects. This work is kindly guided by Dr. Christian Fendt of the MPIA, Heidelberg.Relativistic jets originate in the direct vicinity of the massive black holes residing in the centers of most galaxies. Either they are launched as an initially sonic wind of the accretion disk or they originate directly in the black holes ergosphere transforming rotational energy into high Lorentz factors via magneto hydrodynamic processes.
Here are two movies of RMHD simulations perfomed with the PLUTO code.
Relativistic disk wind in a hourglass magnetosphere. Color gradient indicates the vertical velocity in units of c.
Contours are: Field lines (white); electric current lines (green); light cylinder (blue).
Contours are: Field lines (white); electric current lines (green); light cylinder (blue).
As the previous one but for a highly inclined split monopole geometry.
The field-lines are pushed back into the disk surface and hence no steady state is established.
We generally find that winds from ADAF-like disk coronae produce well collimated outflows but are to heavy to produce highly relativistic flows via the Blandford-Payne process. For more information please have a look at our recent paper.
Three dimensional jet stability
The overwhelming stability of relativistic jets that even propagate out of the host galaxy still puzzles todays astrophysicists. Using three-dimensional simulations with high spatial resolution we try to find the criteria leading to jet disruption or stabilization.Here is a movie from a simulation of a jet-propagation showing the wiggling of the jet head.
Return current density in a simulation of a toroidal jet using AMRVAC. The
jet radius is resolved with 19 grid cells in a domain with
effectively
192 x 192 x 1920 cells. A high effective resolution
was realized using four grid levels in an adaptive mesh.
The simulation was run on 1024 cores of the JADE supercomupter using 50K
cpu hours.
Radiation transport in the simulated AGN core
The radio luminosity (and presumably the optical contiuum as well) of active galactic nuclei originates in the synchotron radiation of relativistic electrons in the helical field of the magnetosphere. We solve the radiation transport due to Synchotron emission and self-absorption in the steady state of the RMHD simulations shown before.Below is a simple synchotron raycasting through a jet formation simulation with peak Lorentz-factor of 4.5.
Radio map from Synchotron emission of the RMHD
snapshot. The x-axis extends over 600 Schwarzschild radii.
left: For various inclination angles. Most flux originates in
the ADAF-like disk corona. Due to relativistic
Doppler-beaming, an increased flux in the approaching part
of the rotating magnetosphere is observed.
right: Scanning over the frequency domain at a given
inclination. The jet becomes optically thin at roughly 86GHz.
Stellar dynamics with central black holes
Tidal disruption rate in galactic nuclei
In my diploma thesis i investigate the tidal disruption rate of stars in orbit around massive black holes. The thesis was kindly supervised by Dr. R. Spurzem of ARI (Heidelberg).Once a star enters the tidal radius of the central black hole, it emmits an X-ray flare while part of its mass is being accreted to the hole. We have performed direct nbody simulations with the MPI-parallel code Nbody6++ on the various supercomputer sites that are part of the DEISA project. We implemented particle disruption to study the coupling between the dynamical and the relaxtational timescales (the classical losscone problem) by making no limiting assumptions on the stellar dynamics.
Our simulations confirm classical estimates of the losscone diffusion and predict a stellar disruption event in the Milkyway once every 10 000 years.
A central massive black hole imprints a clear signature onto the stellar system. The density profile approaches a Bahcall-Wolf cusp (slope -7/4) and the cluster expands. Here are two plots showing the cluster evolution and the disruption rate.
left: Evolution of the density profile of a stellar system with central black
hole. Symbols are: Influence radius (diamond), critical
radius (triangle), wandering radius (square).
right: Comparison of measured and theoretical expectance of the disruption rate. The disruption rate follows the peak losscone flux.
For more information have a look at my diploma thesis.
right: Comparison of measured and theoretical expectance of the disruption rate. The disruption rate follows the peak losscone flux.
Binary black hole hardening in triaxial galaxies
In contrast to spherical systems, the disruption- or binary hardening rate in triaxial systems can be significantly increased. Here, a new family of orbits, the box-orbits pass by the center every orbital period. Also due to the lack of symmetry, chaotic orbits are allowed. This presents a mechanism to effectively merge binary black holes in collisionless systems.
left: Ordinary z-loop orbit.
right: (Thin?) box orbit in a triaxial galaxy with central black hole. Resonant box orbits avoid the direct vicinity of the (binary-) black hole. They do not contribute to the hardening of the black holes.
right: (Thin?) box orbit in a triaxial galaxy with central black hole. Resonant box orbits avoid the direct vicinity of the (binary-) black hole. They do not contribute to the hardening of the black holes.
Oliver Porth
porth @ mpia.de
Max-Planck-Institut für Astronomie
Königstuhl 17
D-69117 Heidelberg
Office 133
Phone: (+49) 6221 528-315