We reconstruct the dark energy density \rho _ { X } ( z ) 
as a free function from current type Ia supernova ( SN Ia ) data ( Tonry et al .
2003 ; Barris et al .
2003 ; Knop et al .
2003 ) , together with the Cosmic Microwave Background ( CMB ) shift parameter from CMB data ( WMAP , CBI , and ACBAR ) , and the large scale structure ( LSS ) growth factor from 2dF galaxy survey data .
We parametrize \rho _ { X } ( z ) as a continuous function , given by interpolating its amplitudes at equally spaced z values in the redshift range covered by SN Ia data , and a constant at larger z ( where \rho _ { X } ( z ) is only weakly constrained by CMB data ) .
We assume a flat universe , and use the Markov Chain Monte Carlo ( MCMC ) technique in our analysis .
We find that the dark energy density \rho _ { X } ( z ) is constant for 0 \lesssim z \lesssim 0.5 and increases with redshift z for 0.5 \lesssim z \lesssim 1 at 68.3 % confidence level , but is consistent with a constant at 95 % confidence level .
For comparison , we also give constraints on a constant equation of state for the dark energy .

Flux-averaging of SN Ia data is required to yield cosmological parameter constraints that are free of the bias induced by weak gravitational lensing ( ) .
We set up a consistent framework for flux-averaging analysis of SN Ia data , based on ( ) .
We find that flux-averaging of SN Ia data leads to slightly lower \Omega _ { m } and smaller time-variation in \rho _ { X } ( z ) .
This suggests that a significant increase in the number of SNe Ia from deep SN surveys on a dedicated telescope ( ) 
is needed to place a robust constraint on the time-dependence of the dark energy density .