The partition function of the two-matrix model as an isomonodromic τ function

We consider the Itzykson–Zuber–Eynard–Mehta two-matrix model and prove that the partition function is an isomonodromic τ function in a sense that generalizes that of Jimbo et al. [ “Monodromy preserving deformation of linear ordinary differential equations with rational coefficients,” Physica D 2, 306 (1981)]. In order to achieve the generalization we need to define a notion of τ function for isomonodromic systems where the adregularity of the leading coefficient is not a necessary requirement.