Thermal index Γ_th as a function of number density for our virial approximation (blue solid line) and several EOSs from CompOSE (dashed lines). The left and right panels display the nuclear thermal index (Equation (29)) and the full thermal index (Equation (24)), respectively, for a proton fraction Y_p = 0.2 and at a temperature of T = 15 MeV (top) and at a temperature of T = 50 MeV (bottom). Note the different scales in the y-axis of the left and right panels. Horizontal dotted lines represent the values of Γ_th in both the relativistic and nonrelativistic limits.