pub fn x_c(z: f64) -> f64Expand description
Combined critical frequency for photon absorption.
x_c² = x_c_DC² + x_c_BR² (quadrature addition)
Photons with x << x_c are absorbed by DC/BR; x >> x_c survive.
Reference: Chluba (2015), arXiv:1506.06582, Eq. 25