Accretion disc structure in SDSS J1339+1310. Left panel: probability density for the source size in Einstein units (⟨M_*/M_⊙⟩^1/2 cm). The distribution levels off at ∼2 × 10¹⁴ cm since the pixel scale in our magnification patterns is 1.6 × 10¹⁴ cm. All solutions become equally likely below that size scale. Right panel: probability density for the half light radius of the continuum-emission region in SDSS J1339+1310 at λ_rest = 1930 Å assuming an inclination angle cos i = 0.5. The solid black curve was derived by convolving our probability density for ⟨M_*⟩ (right panel of Fig. 7) with that of (see left panel). The dashed black curve assumed a uniform prior on the mean mass of a lens galaxy star 0.1 ≤ ⟨M_*⟩ ≤ 1.0. The solid green curve shows the result without the mass prior scaled to 2500 Å for comparison to other models. The Schwarzschild radius r_Sch = 2GM_BH/c² and ISCO at 3r_Sch are plotted for reference.