{c}
{title2=$y=\frac{\sigma_T}{m_ec^2}\int n_ek_BT_e\,d\ell$}

The <Compton y parameter> is the <Thomson scattering> <optical depth> weighted by the dimensionless <Electron> <temperature>. It measures the integrated electron <pressure> along a <line of sight>. When hot, nonrelativistic electrons scatter much colder <photons>, the first-order radiation energy gain is $4y$ times the incident energy. The spectrum is redistributed while the total <photon number density> is conserved locally by scattering.


 Compton y parameter (source code)