Equation of state of tracker fields

We derive the equation of state of tracker fields, which are typical examples of freezing quintessence (quintessence with the equation of state approaching toward -1), taking into account of the late-time departure from the tracker solution due to the nonzero density parameter of dark energy Ωφ. We calculate the equation of state as a function of Ωφ for constant Γ=VV''/(V')2 (during matter era) models. The derived equation of state contains a single parameter, w(0), which parametrizes the equation of state during the matter-dominated epoch. We derive observational constraints on w(0) and find that observational data are consistent with the cosmological constant: -1.11<w(0)<-0.96(1σ).