  We use the Allen et al. (2008) galaxy cluster gas mass fraction versus
redshift data to constrain parameters of three different dark energy models: a
cosmological constant dominated one ($\Lambda$CDM); the XCDM parameterization
of dark energy; and a slowly-rolling scalar field model with inverse-power-law
potential energy density. (Instead of using the Monte Carlo Markov Chain
method, when integrating over nuisance parameters we use an alternative method
of introducing an auxiliary random variable.) The resulting constraints are
consistent with, and typically more constraining than, those derived from other
cosmological data. A time-independent cosmological constant is a good fit to
the galaxy cluster data, but slowly evolving dark energy cannot yet be ruled
out.
