Diagram of a flat, geometrically thin accretion disk formed around the compact object in the center (point O). A source of energetic gamma rays lies above the disk seen at an angle ψ by a distant observer. Gamma rays and photons from the unit surface of the disk dS (centered around point M) interact at the point P with an angle θ₀. Point Q is the projection of point P in the disk plane. All the other geometrical quantities referred to in the text for the calculation of the gamma-ray optical depth are also shown here.