Thermal relaxation and the complete set of second-order transport coefficients for the unitary Fermi gas from kinetic theory

We compute the complete set of second-order transport coefficients of the unitary Fermi gas, a dilute gas of spin-1/2 particles interacting via an $s$-wave interaction tuned to infinite scattering length. The calculation is based on kinetic theory and the Chapman-Enskog method at second order in the Knudsen expansion. We take into account the exact two-body collision integral. We extend previous results on second-order coefficients related to shear stress by including terms related to heat flow and gradients of the fugacity. We confirm that the thermal relaxation time is given by the simple estimate ${\ensuremath{\tau}}_{\ensuremath{\kappa}}=\ensuremath{\kappa}m/({c}_{P}T)$ even if the full collision kernel is taken into account. Here, $\ensuremath{\kappa}$ is the thermal conductivity, $m$ is the mass of the particles, ${c}_{P}$ is the specific heat at constant pressure, and $T$ is the temperature.