  We cross-correlate the new 3 year Wilkinson Microwave Anistropy Probe (WMAP)
cosmic microwave background (CMB) data with the NRAO VLA Sky Survey (NVSS)
radio galaxy data, and find further evidence of late integrated Sachs-Wolfe
(ISW) effect taking place at late times in cosmic history. Our detection makes
use of a novel statistical method \cite{Baldi et al. 2006a, Baldi et al. 2006b}
based on a new construction of spherical wavelets, called needlets. The null
hypothesis (no ISW) is excluded at more than 99.7% confidence. When we compare
the measured cross-correlation with the theoretical predictions of standard,
flat cosmological models with a generalized dark energy component parameterized
by its density, $\omde$, equation of state $w$ and speed of sound $\cs2$, we
find $0.3\leq\omde\leq0.8$ at 95% c.l., independently of $\cs2$ and $w$. If
dark energy is assumed to be a cosmological constant ($w=-1$), the bound on
density shrinks to $0.41\leq\omde\leq 0.79$. Models without dark energy are
excluded at more than $4\sigma$. The bounds on $w$ depend rather strongly on
the assumed value of $\cs2$. We find that models with more negative equation of
state (such as phantom models) are a worse fit to the data in the case $\cs2=1$
than in the case $\cs2=0$.
