Comparison of Kerr and dilaton black hole shadows: Impact of non-thermal emission

Jan Röder (Institut für Theoretische Physik, Goethe -Universität Frankfurt)

With the Event Horizon Telescope, a very long baseline interferometry (VLBI) array, both temporal and spatial event horizon-scale resolutions needed to observe super-massive black holes were reached for the first time. Current open questions revolve around the type of compact object in the Galactic Center, plasma dynam- ics around it and emission processes at play. The main goal of this thesis is to assess whether it is possible to distinguish between two spacetimes by means of synthetic imaging, under the aspect of different emission models. Extending the studies conducted in the pioneering work of Mizuno et al. 2018, general relativis- tic radiative transfer (GRRT) calculations are carried out on general relativistic magneto-hydrodynamics (GRMHD) simulations of a Kerr and of a non-rotating dilaton black hole. The systems are matched at the innermost stable circular orbit, and both black holes are initially surrounded by a torus in hydrostatic equilibrium with a weak poloidal magnetic field. In order to investigate the plasma dynam- ics, GRMHD simulations were carried out using the “Black Hole Accretion Code” (BHAC). In the literature the ratio between the temperatures of simulated ions and radiating electrons is often taken to be a constant, while in reality it is ex- pected to depend on plasma properties. In radiative post-processing with the code “Black Hole Observations in Stationary Spacetimes” (BHOSS) the temperature ra- tio was therefore parametrized. Additionally, in the jet wall, electrons are believed to be accelerated and should therefore be modeled with non-thermal electrons. To this end, both thermal and non-thermal electron energy distribution functions were employed. Lastly, images were reconstructed from synthetic VLBI data with the “eht-imaging” Python package to study how the effects of the emission models carry over to an observational environment. The most impactful result is the effect of the parameter Rhigh in the temperature ratio parametrization, splitting source structures into torus– and jet dominated configurations. Non-thermal emission turns out to be negligible at the field of view used and for the region it is applied in. Hence, given the present observational capabilities, it is unlikely that it is possible to distinguish spacetimes in observations. The striking visual differences are due to the difference in rotation between the black holes. In synthetic VLBI images, even the difference in shadow size is lost for most configurations. The situation may be improved in the future by a better VLBI array.

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