  We propose a phenomenological unified model for dark matter and dark energy
based on an equation of state parameter $w$ that scales with the $\arctan$ of
the redshift. The free parameters of the model are three constants:
$\Omega_{b0}$, $\alpha$ and $\beta$. Parameter $\alpha$ dictates the transition
rate between the matter dominated era and the accelerated expansion period. The
ratio $\beta / \alpha$ gives the redshift of the equivalence between both
regimes. Cosmological parameters are fixed by observational data from
Primordial Nucleosynthesis (PN), Supernovae of the type Ia (SNIa), Gamma-Ray
Bursts (GRB) and Baryon Acoustic Oscillations (BAO). The calibration of the 138
GRBs events is performed using the 580 SNIa of the Union2.1 data set and a new
set of 79 high-redshift GRBs is obtained. The various sets of data are used in
different combinations to constraint the parameters through statistical
analysis. The unified model is compared to the $\Lambda$CDM model and their
differences are emphasized.
