The profiles of lines from a flat. relativistic disk around a Schwarzschild black hole are computed with a rigorous treatment of the energy shifts and photon trajectories. It is found that, unless the disk is optically thin, further structures emerge between the two Doppler horns, at high ( greater than or similar to 80-degrees) inclination angles. These structures are due to a geodesics focusing effect on photons emitted from the ''far'' side of the disk. Their shape and relative intensity are a function of the radial dependence and angular distribution of the line emissivity. Since these structures are due to a purely general relativistic effect, they could provide an unambiguous signature of a relativistic disk in Active Galactic Nuclei and in X-ray binaries.
Matt, G., Perola, G.c., Stella, L. (1993). MULTIPLE-PEAKED LINE-PROFILES FROM RELATIVISTIC DISKS AT HIGH INCLINATION ANGLES. ASTRONOMY & ASTROPHYSICS, 267(2), 643-650.