Using space-VLBI to probe gravity around Sgr A*
  The Event Horizon Telescope (EHT) will soon provide the first high-resolution
images of the Galactic Centre supermassive black hole (SMBH) candidate
Sagittarius A* (Sgr A*), enabling us to probe gravity in the strong-field
regime. Besides studying the accretion process in extreme environments, the
obtained data and reconstructed images could be used to investigate the
underlying spacetime structure. In its current configuration, the EHT is able
to distinguish between a rotating Kerr black hole and a horizon-less object
like a boson star. Future developments can increase the ability of the EHT to
tell different spacetimes apart. We investigate the capability of an advanced
EHT concept, including an orbiting space antenna, to image and distinguish
different spacetimes around Sgr A*. We use GRMHD simulations of accreting
compact objects (Kerr and dilaton black holes, as well as boson stars) and
compute their radiative signatures via general relativistic radiative transfer
calculations. To facilitate comparison with upcoming and future EHT
observations we produce realistic synthetic data including the source
variability, diffractive and refractive scattering while incorporating the
observing array, including a space antenna. From the generated synthetic
observations we dynamically reconstructed black hole shadow images using
regularised Maximum Entropy methods. We employ a genetic algorithm to optimise
the orbit of the space antenna with respect to improved imaging capabilities
and u-v-plane coverage of the combined array (ground array and space antenna
and developed a new method to probe the source variability in Fourier space.
The inclusion of an orbiting space antenna improves the capability of the EHT
to distinguish the spin of Kerr black holes and dilaton black holes based on
reconstructed radio images and complex visibilities.
