Determining the equation of state of dark energy from angular size of compact radio sources and X-ray gas mass fraction of galaxy clusters

Using recent measurements of angular size of high-z milliarcsecond compact radio sources compiled by Gurvits et al. (CITE) and X-ray gas mass fraction of galaxy clusters published by Allen et al. (CITE, CITE), we explore their bounds on the equation of state, , of the dark energy, whose existence has been congruously suggested by various cosmological observations. We relax the usual constraint , and find that combining the two databases yields a nontrivial lower bound on . Under the assumption of a flat universe, we obtain a bound at 95.4% confidence level. The 95.4% confidence bound goes to when the constraint is imposed.