Nominal thermodynamic temperature in nonequilibrium kinetic Ising models.

We show that a nominal temperature can be consistently and uniquely defined everywhere in the phase diagram of large classes of nonequilibrium kinetic Ising spin models. In addition, we confirm that, at critical points, the large-time "fluctuation-dissipation ratio" X( infinity ) is a universal amplitude ratio, and find in particular X( infinity ) approximately 0.33(1) and X( infinity )=1 / 2 for the magnetization in, respectively, the two-dimensional Ising and voter universality classes.