  The nature of the dark energy is still a mystery and several models have been
proposed to explain it. Here we consider a phenomenological model for dark
energy decay into photons and particles as proposed by Lima (J. Lima, Phys.
Rev. D 54, 2571 (1996)). He studied the thermodynamic aspects of decaying dark
energy models in particular in the case of a continuous photon creation and/or
disruption. Following his approach, we derive a temperature redshift relation
for the CMB which depends on the effective equation of state $w_{eff}$ and on
the "adiabatic index" $\gamma$. Comparing our relation with the data on the CMB
temperature as a function of the redshift obtained from Sunyaev-Zel'dovich
observations and at higher redshift from quasar absorption line spectra, we
find $w_{eff}=-0.97 \pm 0.034$, adopting for the adiabatic index $\gamma=4/3$,
in good agreement with current estimates and still compatible with
$w_{eff}=-1$, implying that the dark energy content being constant in time.
