Thermal index of neutron matter as a function of density for different temperatures. We show results from T = 5 MeV to T = 45 MeV with increments of 10 MeV. Dotted (dashed) lines represent the first-order (second-order) expansion of Equation (14). Solid lines are the results of the ratio of Equation (3) with second-order pressure in Equation (5) and energy densities in Equation (8). The shaded area shows an estimate for the error in the expansion, based on incorporating the third-order correction ﹩\pm {b}_{n}^{(3)}﹩. The dotted gray shows the value of Γ_th = 5/3 in the noninteracting limit, for reference. We only show results with fugacities below z < 0.3.