Black hole parameter estimation with synthetic very long baseline interferometry data from the ground and from space

¹ Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
e-mail: freek.roelofs@cfa.harvard.edu
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³ Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
⁴ Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Straße 1, 60438 Frankfurt am Main, Germany
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Tsung-Dao Lee Institute and School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
⁷ Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, 550 W 120th St., New York, NY 10027, USA
⁸ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
⁹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
¹⁰ School of Mathematics, Trinity College, Dublin 2, Ireland
¹¹ Frankfurt Institute for Advanced Studies, Ruth-Moufang-Straße 1, 60438 Frankfurt, Germany

Received: 22 October 2020
Accepted: 19 March 2021

Abstract

Context. The Event Horizon Telescope (EHT) has imaged the shadow of the supermassive black hole in M 87. A library of general relativistic magnetohydrodynamics (GMRHD) models was fit to the observational data, providing constraints on black hole parameters.

Aims. We investigate how much better future experiments can realistically constrain these parameters and test theories of gravity.

Methods. We generated realistic synthetic 230 GHz data from representative input models taken from a GRMHD image library for M 87, using the 2017, 2021, and an expanded EHT array. The synthetic data were run through an automated data reduction pipeline used by the EHT. Additionally, we simulated observations at 230, 557, and 690 GHz with the Event Horizon Imager (EHI) Space VLBI concept. Using one of the EHT parameter estimation pipelines, we fit the GRMHD library images to the synthetic data and investigated how the black hole parameter estimations are affected by different arrays and repeated observations.

Results. Repeated observations play an important role in constraining black hole and accretion parameters as the varying source structure is averaged out. A modest expansion of the EHT already leads to stronger parameter constraints in our simulations. High-frequency observations from space with the EHI rule out all but ∼15% of the GRMHD models in our library, strongly constraining the magnetic flux and black hole spin. The 1σ constraints on the black hole mass improve by a factor of five with repeated high-frequency space array observations as compared to observations with the current ground array. If the black hole spin, magnetization, and electron temperature distribution can be independently constrained, the shadow size for a given black hole mass can be tested to ∼0.5% with the EHI space array, which allows tests of deviations from general relativity. With such a measurement, high-precision tests of the Kerr metric become within reach from observations of the Galactic Center black hole Sagittarius A*.