Towards direct imaging and orbital parameter estimation of supermassive black hole binaries with spaceborne VLBI
Direct observation of the orbital motion of a sub-parsec supermassive black hole binary (SMBHB) would provide the first conclusive electromagnetic proof of such systems existing. Widely considered to be the source of gravitational waves, binaries are expected to form as a natural consequence of galactic mergers, and determining the processes that drive their evolution is essential to understanding cosmological evolution. In this work, we evaluate the prospects of using ground and spaceborne Very Long Baseline Interferometry (VLBI) to observe SMBHBs and estimate their orbital parameters. The Black Hole Explorer (BHEX) is considered the primary case study. Achieving unprecedented resolution, BHEX will provide access to a new volume of binary parameter space, potentially enabling the first confident detection of an SMBHB. We present an orbit-fitting approach that uses relative astrometry and Bayesian dynamic nested sampling, and demonstrate its efficacy on a set of example binary systems. For simulating observations, we use a binary image model based on post-Newtonian orbit propagation and find that for BHEX, binary detection requires a total flux density of 40 mJy and a minimum separation of $\sim$2 $\mu$as. With three annual observations, BHEX could constrain the semi-major axis and the eccentricity of binaries with orbital periods of $\leq$10 years to within 0.06 dex of the true values under specific noise assumptions. We have also evaluated the benefits provided by BHEX for binary detection compared to ground-only observations by arrays such as the next generation Event Horizon Telescope (ngEHT). Finally, we constrained the requirements of a future spaceborne VLBI system capable of performing a statistically significant survey of SMBHBs.