Alumni 2024
Zhiqiu Huang (China) 23.05.2024
Gamma-ray Bursts and Implications for Particle Acceleration at Ultra-relativistic Shocks ( thesis pdf, 20 MB)
Recent TeV detections of gamma-ray burst afterglows offer new insights into particle acceleration at relativistic shocks. Kinetic simulations have improved our understanding of shock microphysics, enhancing models of particle acceleration relevant to afterglows. We explore scenarios for determining the maximum achievable energy, comparing our findings with data from the H.E.S.S. source, GRB 190829A. This comparison reveals a tension between observations and theoretical expectations. Motivated by this, we developed a Monte Carlo code to revisit acceleration theory for relativistic shocks in uniform and non-uniform magnetic field configurations. In uniform fields, we demonstrate that acceleration requires only strong scattering on one side of the shock. Analytic solutions confirm this conclusion. For non-uniform fields, we consider a cylindrical magnetic-field structure typical of astrophysical jets. We find that curvature drifts enable repeated shock-crossings for particles of favourable charge, and neglecting losses extends the maximum energy to the system's confinement limit. These results challenge the misconception that ultra-relativistic shocks cannot serve as effective accelerators, offering a fresh perspective on relativistic shock acceleration. The findings suggest new features on maximum achievable energy and spectral index, indicating the need to revisit current knowledge on relativistic shocks. This could open promising avenues for producing ultra-high energy cosmic rays.
Supervisor: Brian Reville (MPIK)
Javier Moran Fraile (Spain) 30.04.2024
Simulating the dynamical interaction of white dwarf stars in binaries ( thesis pdf, 25 MB)
Binary stellar interactions that take place on dynamical timescales are some of the most challenging processes to model in astronomy, and are best described by multidimensional, multi-physics simulations. The focus of this thesis is on the numerical modeling of some of the least explored stellar interactions, with a spe- cial emphasis on those involving white dwarfs. This work presents three different studies using three-dimensional hydrodynamic simulations. In the first place, the emission of gravitational waves during common-envelope events is studied, esti- mating the chances for their detection with future space-based detectors. Secondly, the tidal disruption of a white dwarf by a neutron star is studied, showing how the accurate modeling of these events requires the inclusion of a multitude of physi- cal processes including magnetic fields, nuclear reactions and neutrino emission. Finally, through a third simulation, it is shown how mergers between low-mass white dwarfs, with total masses substantially below the Chandrasekhar limit, can lead to thermonuclear explosions under the right conditions. The results of this thesis stress how dynamical interactions between stars can produce a multitude of bright transients, and how the use of advanced multidimensional, multi-physics codes for their modeling will help improve our understanding of the physics and processes involved.
Supervisor: Friedrich Roepke (HITS)
Toni Peter (Germany) 23.04.2024
Understanding the Era of Reionization via Numerical Methods for Radiative Transfer ( thesis pdf, 22 MB)
The goal of this thesis are numerical studies of the era of reionization, which took place at about a 150 million years to a billion years after the Big Bang. Since reionization is a process driven by radiation, a major fraction of this work is dedicated to numerical methods for radiative transfer. In particular, we develop the Sweep method, which allows us to study reionization within large cosmological simulations. We begin by in- troducing the basics of the Sweep algorithm and its implementation in the simulation code Arepo. We discuss the motivation behind it, how it integrates with the rest of Arepo and perform a number of tests to assess its performance and physical accuracy. We find that the Sweep method does not only produce physically accurate results, but does so in a very efficient manner, even when applied to large simulations on a large number of processors. We then proceed by introducing the standalone radiative trans- fer postprocessing code Subsweep in which we add a variety of improvements to the original Sweep method, in particular the addition of sub-timesteps. We perform a number of additional tests to verify that Subsweep correctly solves a number of physi- cal problems and show that sub-timesteps can drastically improve performance of the Sweep algorithm when applied to problems with heterogeneous environments without sacrificing accuracy. Finally, we apply Subsweep to the cosmological simulation suite TNG in order to recreate the era of reionization in the TNG universe. We find that Subsweep allows us to study the spatial structure of reionization in detail and that we can reproduce the observational constraints on the history of reionization reasonably well.
Supervisor: Ralf Klessen (ITA)
Bastian Alejandro Reinoso Reinoso (Chile) 15.04.2024
Formation of massive black hole seeds through runaway stellar collisions and gas accretion in dense stellar systems ( thesis pdf, 30 MB)
The goal of this work is to gain a better understanding of the processes that lead to the formation of massive black hole seeds in the early Universe, in order to provide insights into the rapid emergence of the highest redshift quasars. Two different seeding mechanisms were studied via numerical simulations. The first mechanism explores the onset of runaway stellar collisions in dense clusters of Population III stars, focusing on understanding the role of an external potential for modelling the gas during the embedded phase. Stellar collision rates were also explored in a similar environment with the goal of confronting analytic estimates with numerical simulations. The study of this seeding mechanism demonstrates the plausibility of forming black hole seeds with > 1000 M⊙ through runaway stellar collisions that produce very massive stars. Furthermore, an analytic model for estimating the number of collisions in dense star clusters is presented, and the identification of a new collision channel involving perturbations on binary stars is reported. The second seeding mechanism explored in this work deals with the emergence of supermassive stars through the interplay of gas accretion and stellar collisions in environments resembling collapsed gas clouds in atomic cooling halos. The numerical implementation developed in this work allowed for a self-consistent treatment of stellar and gas dynamics for the exploration of this mechanism. The results show that the emergence of supermassive stars with 10^4 M⊙ is inevitable and a binary system of supermassive stars is the outcome in one third of the cases. This thesis concludes by summarizing and discussing the results found in these studies and commenting on the future work needed to improve the models presented here.
Supervisor: Ralf Klessen (ITA)
Francesco Conte (Italy) 01.02.2024
Gamma-ray emission and absorption in the inner few parsecs of the Galactic Centre ( thesis pdf, 15 MB)
Located 8 kpc away, the Galactic Centre is a rich environment for observing non-thermal phenomena such as a supermassive black hole, potential dark matter accumulations, supernova remnants, pulsar wind nebulae, clustered massive stars, and many more. It is a key target for both operational and next-generation TeV observatories like H.E.S.S., MAGIC, and CTA. Current telescopes, limited by a full-width half-maximum of 5 arcminutes, struggle to pinpoint the nature of gamma-ray sources amidst the Galactic Centre’s complexity. UV/visible observations are also compromised due to dust absorption and infrared re-emission. However, this study leverages the infrared radiation’s ability to absorb high-energy photons, using a model of the infrared field for spatial and spectral gamma-ray analyses. In this thesis I present the first 3D model for the infrared radiation field in the inner few parsecs. By studying the high-energy absorption, I find that if the central gamma-ray source and the large scale gamma-ray emission share the same cosmic-ray accelerator, then the central emitter is a ring of outer radius 2.5 pc that CTA will see as an extended source. In that case, the diffuse gamma-ray emission is expected to show a turn-off around 20 TeV rather than a power-law to 100 TeV.
Supervisor: Richard Tuffs (MPIK)
