Posterior distribution of the general relativistic Bayesian fit. Here the BH mass-to-distance ratio M/D is expressed in ×10⁴ M_⊙ Mpc⁻¹, x₀ is expressed in mas, and v_rec is in km s⁻¹. Contour levels correspond to 1σ and 2σ confidence regions. We have considered flat priors for M/D :  [0, 200] × 10⁴ M_⊙ Mpc⁻¹, x₀ :  [−1.5, 1.5] mas, and z_rec :  [0, 0.03] (v_rec :  [0, 9000] km s⁻¹).