Future Prospects for Constraining Black Hole Spacetime

The Event Horizon Telescope (EHT) has ushered in a new era for observing the strong gravitational field and testing general relativity. Recent observations of the horizon scale images of the supermassive black hole M87* at the nearby galaxy M87 provide direct information in the vicinity of the black hole. The comparisons of the EHT observations and theoretical simulations enable us to constrain the spacetime, while it is not easy to decompose into the accretion flow properties and spacetime information because the radiation complexly depends on their effects. The next generation EHT (ngEHT) projects are expected to explore the astrophysical jet dynamics around the black hole, one of the most promising topics for extracting spacetime information.
We demonstrate that the black hole spacetime can be constrained by focusing on the dynamics of the jet under the observational environments of ngEHT projects. We utilize the three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulation of magnetically arrested disks (MADs) with the numerical code BHAC. The simulated jet movies are calculated with the general-relativistic radiative transfer (GRRT) scheme of the numerical code BHOSS. We introduce the hybrid thermal–non-thermal particle distribution function optimized by comparing observational and simulated continuum spectra of M87* provided in Fromm et al. (2021). Around the jet base, the launched jet has intermittent wave structures, which propagate with a relativistic time scale.
The wave velocity and time development can be extracted from the simulated movies and show monotonic dependence on the black hole spacetime. We finally perform the synthetic observation of M87* and show the detectability of velocity features and the black hole spacetime information under the expected ngEHT observations.