Posterior probability densities of the radius of the C IV BLR in J1138. The BLR radius is expressed in Einstein radius units, with r_E = 11.7 light-days for ℳ = 0.3 ℳ_⊙. The top panel probabilities are computed with M(C IV) = 0.90, and the bottom panel probabilities with M(C IV) = 1.05. Left panels: probability densities of the half-light radius, r_1/2, obtained with two magnification maps that were computed with different fractions of compact objects, κ_⋆/κ = 7% (black) and κ_⋆/κ = 20% (blue), and after marginalizing over the two maps (red). Middle panels: same as the left panels but for the flux-weighted mean radius, r_mean. Right panels: comparison of the probability densities, marginalized over the two maps, of the half-light radius (red and blue curves) and the flux-weighted mean radius (magenta and black curves), computed with constraints from the continuum source magnification (red and magenta curves) and without this constraint (blue and black curves).