Equation of state for two flavor QCD at N_t=6

We calculate the two flavor equation of state for QCD on lattices with lattice spacing a = (6T)^-1 and find that cutoff effects are substantially reduced compared to an earlier study using a = (4T)^-1. However, it is likely that significant cutoff effects remain. We fit the lattice data to expected forms of the free energy density for a second order phase transition at zero-quark mass, which allows us to extrapolate the equation of state to m_q=0 and to extract the speed of sound. We find that the equation of state depends weakly on the quark mass for small quark mass.