Investigating the observational appearance of black holes and other compact objects with GRMHD

The aim of the project is to study the observational appearance of black holes and other compact objects by combining numerical simulations and ray-tracing. The project will be carried out within the NCN SONATINA grant. Its main goal is to determine whether present and future observations of the Event Horizon Telescope (EHT) can distinguish a standard rotating black hole from other compact objects predicted in alternative models of gravity.
The recent success of the EHT has opened a new way to test gravity in the strong-field regime. Observations of M87 and Sgr A* provide direct information about plasma emission very close to the central compact object. However, most theoretical image libraries used in the interpretation of such observations are based on the Kerr black hole(black hole with mass and spin) model. This limits our ability to test whether the observed sources are consistent with the standard picture or whether they may show signatures of exotic compact objects.
The project will address this problem by studying accretion flows in both Kerr and non-Kerr spacetimes. First, we will construct analytical models of equilibrium accretion structures (eg, Mishra et al. 2024). These models will provide physically motivated initial conditions for numerical simulations. Next, we will perform general relativistic magnetohydrodynamic (GRMHD) simulations with the KHARMA code with different space-time metric backgrounds. These simulations describe the time evolution of hot magnetized plasma in strong gravity. They allow us to study the structure of the accretion flow, magnetic-field evolution, and possible outflows or jets. In the final stage, we will apply ray-tracing with the ipole code in order to construct synthetic images that can be compared directly with the EHT images.
A major scientific objective is to determine how the underlying spacetime geometry affects observable properties. In particular, we will study the size and morphology of the central dark region, the brightness asymmetry of the image, and the time variability of the emission.
The project will include models of standard black holes as well as selected non-Kerr compact objects, including more exotic cases such as naked singularities. It will provide synthetic image templates for Kerr and non-Kerr compact objects in order to enable future discrimination between different alternative metrics of the observed objects. The requested calculation period is aligned with the duration of the NCN SONATINA project under which this research is carried out.