pub fn dc_gaunt_factor(x: f64, theta_z: f64) -> f64Expand description
DC Gaunt factor in the soft photon limit with relativistic corrections.
g_dc(x, θ_z, θ_e) ≈ (I₄^pl / (1 + 14.16 θ_z)) · H_dc(x)
where I₄^pl = 4π⁴/15 and H_dc is the high-frequency suppression.
The factor (1 + 14.16 θ_z)⁻¹ is the leading-order relativistic correction from thermal averaging (Chluba+ 2007), evaluated at the photon temperature θ_z. Strictly the thermal average runs over the electron distribution, so θ_e would be the cleaner choice when T_e ≠ T_z (injection-driven heating); however CS2012 parametrises DC through θ_z and the difference is <1% for θ_z ≲ 10⁻³ even when |ρ_e−1| ~ 0.1.