= Solution
After <thermal decoupling in cosmology>, the collisionless relativistic species has $T_Xa=\text{constant}$. The still-coupled plasma separately conserves entropy, so $g_{*s}T_\gamma^3a^3$ remains constant. At decoupling $T_X=T_\gamma$, while at late times the coupled plasma contains only the two photon polarizations. Therefore the <temperature of a decoupled relativistic relic> is
$$
\boxed{\frac{T_X}{T_\gamma}=\left(\frac{2}{g_*}\right)^{1/3}}.
$$
Here $g_*$ is understood as the effective entropy degrees of freedom in the plasma that remains coupled after $X$ decouples.
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