Constraining the dark energy with galaxy cluster x-ray data

The equation of state characterizing the dark energy component is constrained by combining Chandra observations of the x-ray luminosity of galaxy clusters with independent measurements of the baryonic matter density and the latest measurements of the Hubble parameter as given by the HST key project. By assuming a spatially flat scenario driven by a ``quintessence'' component with an equation of state pₗ=ₗ, we place the following limits on the cosmological parameters and ₌: (i) -1<~<~-0. 55 and ₌=0. 32-₀. ₀₁₄^+0. 027 (1) if the equation of state of the dark energy is restricted to the interval -1<~<0 (the usual quintessence) and (ii) =-1. 29-₀. ₇₉₂^+0. 686 and ₌=0. 31-₀. ₀₃₄^+0. 037 (1) if violates the null energy condition and assume values below -1 (extended quintessence or ``phantom'' energy). These results are in good agreement with independent studies based on supernovae observations, large-scale structure, and the anisotropies of the cosmic background radiation.