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 pₓ=ωρₓ, we place the following limits on the cosmological parameters ω and Ωₘ: (i) -1<~ω<~-0.55 and Ωₘ=0.32_-0.014+0.027 (1σ) if the equation of state of the dark energy is restricted to the interval -1<~ω<0 (the usual quintessence) and (ii) ω=-1.29_-0.792+0.686 and Ωₘ=0.31_-0.034+0.037 (1σ) if ω 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.