Using space-VLBI to probe gravity around Sgr A^*

¹ Institut für Theoretische Physik, Goethe Universität, Max-von-Laue-Str. 1, 60438 Frankfurt, Germany
e-mail: cfromm@th.physik.uni-frankfurt.de
² Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Tsung-Dao Lee Institute and School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, PR China
⁵ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁶ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010 6500, GL, Nijmegen, The Netherlands
⁷ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
⁸ Dipartimento di Fisica “E. Pancini”, Universitá di Napoli “Federico II”, Via Cinthia, 80126 Napoli, Italy
⁹ INFN Sez. di Napoli, Via Cinthia, 80126 Napoli, Italy
¹⁰ Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, UK
¹¹ Frankfurt Institute for Advanced Studies, Ruth-Moufang-Strasse 1, 60438 Frankfurt, Germany
¹² School of Mathematics, Trinity College, Dublin 2, Ireland

Received: 17 December 2019
Accepted: 18 January 2021

Abstract

Context. The Event Horizon Telescope (EHT) will soon provide the first high-resolution images of the Galactic Centre supermassive black hole candidate Sagittarius A* (Sgr A^*), enabling us to probe gravity in the strong-field regime. In addition to 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, EHT is able to distinguish between a rotating Kerr black hole and a horizon-less object such as a boson star. Future developments can increase the ability of EHT to tell different spacetimes apart.

Aims. We investigate the capability of an advanced EHT concept, including an orbiting space antenna, to image and distinguish different spacetimes around Sgr A^*.

Methods. We used general-relativistic magneto-hydrodynamical simulations of accreting compact objects (Kerr and dilaton black holes as well as boson stars) and computed their radiative signatures via general-relativistic radiative transfer. To facilitate a comparison with upcoming and future EHT observations, we produced 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 employed 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.

Results. The inclusion of an orbiting space antenna improves the capability of EHT to distinguish the spin of Kerr black holes and dilaton black holes based on reconstructed radio images and complex visibilities.