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