Constraints on a variable dark energy model with recent observations

We place, by the maximum likelihood method, constraints on a variable dark energy model with the equation of state $w=w_0/[1+b\ln (1+z)]^2$ using some recent observational data, including the new Sne Ia data from the SNLS, the size of baryonic acoustic oscillation peak from SDSS and the CMB data from WMAP3. We find that the SNLS data favor models with $w_0$ around -1, in contrast to the Gold data set which favors a more negative $w_0$. By combining these three databases, we obtain that $\Omega_m=0.27_{-0.038}^{+0.036}$, $w_0=-1.11_{-0.30}^{+0.21}$ and $b=0.31^{+ 0.71}_{-0.31}$ with $\chi^2=110.4$ at the 95% confidence level. Our result suggests that a varying dark energy model and a crossing of the $w = -1$ line are favored, and the present value of the equation of state of dark energy is very likely less than -1