w and w' of Scalar Field Models of Dark Energy
  Important observables to reveal the nature of dark energy are the equation of state $w$ and its time derivative in units of the Hubble time $w'$. Recently, it is shown that the simplest scalar field models of dark energy (quintessence) occupy rather narrow regions in the $w-w'$ plane. We extend the $w-w'$ plane to $w<-1$ and derive bounds on $w'$ as a function of $w$ for tracker phantom dark energy. We also derive bounds on tracker k-essence. 
Takeshi Chiba