The dark energy component of the cosmic budget is represented by a
self-interacting scalar field. The violation of the null energy condition is
allowed. Hence, such component can also represent a phantom fluid. The model is
tested using supernova type Ia and matter power spectrum data. The supernova
test leads to preferred values for configurations representing the phantom
fluid. The matter power spectrum constraints for the dark energy equation of
state parameter are highly degenerated. In both cases, values for the equation
of state parameter corresponding to the phantom fluid are highly admitted if no
particular prior is used.Comment: Latex file, 12 pages, 12 figures in eps forma
