Schematic diagram of bunches seen in the horizon plane. The unit vector of the LOS is denoted by n , and ϵ _∥ and ϵ _⊥ denote the two polarization components. The opening angle of the emission region is 2φ _t . The yellow shaded area denotes emission cones with 1/γ. In the left panel, 0 < φ _t ≲ 1/γ. In the right panel, 1/γ ≲ φ _t < 10/γ.