Studying the Near-horizon Jet of M87 using Millimeter Balloon-borne VLBI stations

In this paper we present synthetic Very Long Baseline Interferometry (VLBI) observations of M87 between a near-future configuration of the Event Horizon Telescope (EHT) and one or multiple 2.5 m-class balloon-borne VLBI stations on a northern transatlantic flight trajectory launching from Kiruna, Sweden. These balloon-borne VLBI stations have the advantage of operating above 99.5% of the atmosphere and their motion allows for efficient sampling of the uv-plane. Assuming frequency phase transfer between 230 and 86 GHz we run these simulations using a modified version of the ngehtsim software package. Our simulations demonstrate the ability of such a VLBI array to probe M87 at the jet base adding baselines down to 0.2 G$\lambda$ for a single balloon-VLBI station and 0.07 G$\lambda$ for three balloon VLBI stations. Using the Bayesian imaging software Comrade.jl, we demonstrate the recovery of jet structures up to 150 $\mu$as in size for a single balloon and up to 300 $\mu$as in size with three balloons. Finally, we test the improvement in jet image fidelity by computing the normalized cross-correlation (NXCOR) and normalized-root-mean-square error (NRMSE) between the image reconstruction and ground truth. We find a factor of 1.2 improvement in the jet NRMSE for a single balloon and a factor of 1.3 improvement for 3 balloons as compared to EHT-only simulations.