New Constraints on the Variable Equation of State Parameter from X-Ray   Gas Mass Fractions and SNe Ia
  Recent measurements are suggesting that we live in a flat Universe and that its present accelerating stage is driven by a dark energy component whose equation of state may evolve in time. Assuming two different parameterizations for the function $\omega(z)$, we constrain their free parameters from a joint analysis involving measurements from X-Ray luminosity of galaxy clusters and SNe type Ia data. 
J. V. Cunha, L. Marassi and R. C. Santos