Comparison of Supernovae datasets Constraints on Dark Energy

Cosmological measurements suggest that our universe contains a dark energy component. In order to study the dark energy evolution, we constrain a parameterized dark energy equation of state $w(z)=w_0 + w_1 \frac{z}{1+z}$ using the recent observational datasets: 157 Gold type Ia supernovae and the newly released 182 Gold type Ia supernovae by maximum likelihood method. It is found that the best fit $w(z)$ crosses -1 in the past and the present best fit value of $w(0)<-1$ obtained from 157 Gold type Ia supernovae. The crossing of -1 is not realized and $w_0=-1$ is not ruled out in $1\sigma$ confidence level for the 182 Gold type Ia supernovae. We also find that the range of parameter $w_0$ is wide even in $1\sigma$ confidence level and the best fit $w(z)$ is sensitive to the prior of $\Omega_m$.