Two-Field Quintom Models in the w-w' Plane

The w-w' plane, defined by the equation of state parameter for the dark energy and its derivative with respect to the logarithm of the scale factor, is useful to the study of classifying the dynamical dark energy models. In this note, we examine the evolving behavior of the two-field quintom models with w crossing the w=-1 barrier in the w-w' plane. We find that these models can be divided into two categories, type A quintom in which w changes from >-1 to <-1 and type B quintom in which w changes from <-1 to >-1 as the universe expands.