The limit on w' for tachyon dark energy

We get the same degeneracy relation between w₀ and wₐ for the tachyon fields as for quintessence and phantom fields. Our results show that the dynamics of scalar fields with different origins becomes indistinguishable when the equation of state parameter w does not deviate too far away from -1, and the time variation w' satisfies the same bound for the same class of models. For the tachyon fields, a limit on w' exists due to the Hubble damping and we derived the generic bounds on w' for different classes of models. We may distinguish different models in the phase plane of w-w'. The current constraints on w and w' are consistent with all classes of models. We need to improve the constraint on w' by 50% to distinguish different models.