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 p(x) = omegarho(x), we place the following limits on the cosmological parameters omega and Omega(m): (i) -1 less than or equal to omega less than or equal to -0.55 and Omega(m) = 0.32(0.014)(+0.027) (1 sigma) if the equation of state of the dark energy is restricted to the interval -1 less than or equal to omega < 0 (the usual quintessence) and (ii), omega = -1.29(-0.792)(+0.686) and Omega(m) = 031(-0.034)(+0.037) ( 1 sigma) if omega 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.