Effect of gravitational microlensing on broad line emission. The illustrated caustic structure is computed for a (κ,γ) = (0,0.5) Chang-Refsdal lens. The BLR is a Keplerian disk seen nearly face-on, whose size is fixed to r_in = 0.1 r_E. The Einstein radius, r_E, is indicated in the upper panels. The upper panels show the emission of the Keplerian disk, integrated over the line profile, and superimposed on the magnifying structure. The upper right panel is a zoom of a 2 r_E × 2 r_E region. The intermediate series of panels present snapshots of the BLR emission in six bins of Doppler velocities sampling the line profile, superimposed on the magnification map. The microlensed flux is obtained by the multiplication of the snapshots of the BLR surface brightness at different Doppler velocities with the region of the caustic structure sampled by the BLR model. The resulting distorted line profile is plotted as a solid red line in the lower panel.