Polarization-dependent resonant differential cross-section (1/σ_T) dσ₁₂/d cos θ′ as a function of photon energy E = ℏω for a set of incident angles θ and scattered angles θ′. The set of angles is 0, 30, 60, and 90 degrees, with θ changing vertically and θ′ horizontally. The red solid lines are produced from the first order expansion (5b) of Sina (1996), while the black dashed ones are produced from the simplified expression (6b). In all cases, the resonant frequency is given by Eq. (7). The vertical line indicates the resonant frequency ω_r, which for θ ≠ 0 is smaller than ω_c. Here, E_c = ℏω_c = 15.33 keV.