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 was 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. The observational window for w' for w <-1 is not narrow, sigma(w') less than or similar to 6 vertical bar 1+w vertical bar.