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We present constraints on the dark energy equation-of-state parameter,w=P/(rho c^2), using 60 Type Ia supernovae (SNe Ia) from the ESSENCE supernovasurvey. We derive a set of constraints on the nature of the dark energyassuming a flat Universe. By including constraints on (Omega_M, w) from baryonacoustic oscillations, we obtain a value for a static equation-of-stateparameter w=-1.05^{+0.13}_{-0.12} (stat; 1 sigma) +- 0.11 (sys) andOmega_M=0.274^{+0.033}_{-0.020} (stat; 1 sigma) with a best-fit chi^2/DoF of0.96. These results are consistent with those reported by the SuperNova LegacySurvey in a similar program measuring supernova distances and redshifts. Weevaluate sources of systematic error that afflict supernova observations andpresent Monte Carlo simulations that explore these effects. Currently, thelargest systematic currently with the potential to affect our measurements isthe treatment of extinction due to dust in the supernova host galaxies.Combining our set of ESSENCE SNe Ia with the SuperNova Legacy Survey SNe Ia, weobtain a joint constraint of w=-1.07^{+0.09}_{-0.09} (stat; 1 sigma) +- 0.12(sys), Omega_M=0.267^{+0.028}_{-0.018} (stat; 1 sigma) with a best-fitchi^2/DoF of 0.91. The current SN Ia data are fully consistent with acosmological constant.

Observational Constraints on the Nature of Dark Energy: First Cosmological Results from the ESSENCE Supernova Survey