  A rigorous thermodynamic analysis has been done at the apparent horizon of a
spatially flat Friedmann-Lemaitre-Robertson-Walker universe for the
gravitationally induced particle creation scenario with constant specific
entropy and an arbitrary particle creation rate $\Gamma$. Assuming a perfect
fluid equation of state $p=(\gamma -1)\rho$ with $\frac{2}{3} \leq \gamma \leq
2$, the first law, the generalized second law (GSL), and thermodynamic
equilibrium have been studied and an expression for the total entropy (i.e.,
horizon entropy plus fluid entropy) has been obtained which does not contain
$\Gamma$ explicitly. Moreover, a lower bound for the fluid temperature $T_f$
has also been found which is given by $T_f \geq
8\left(\frac{\frac{3\gamma}{2}-1}{\frac{2}{\gamma}-1}\right)H^2$. It has been
shown that the GSL is satisfied for $\frac{\Gamma}{3H} \leq 1$. Further, when
$\Gamma$ is constant, thermodynamic equilibrium is always possible for
$\frac{1}{2}<\frac{\Gamma}{3H} < 1$, while for $\frac{\Gamma}{3H} \leq
\text{min}\left\lbrace \frac{1}{2},\frac{2\gamma -2}{3\gamma -2} \right\rbrace$
and $\frac{\Gamma}{3H} \geq 1$, equilibrium can never be attained.
Thermodynamic arguments also lead us to believe that during the radiation
phase, $\Gamma \leq H$. When $\Gamma$ is not a constant, thermodynamic
equilibrium holds if $\ddot{H} \geq \frac{27}{4}\gamma ^2 H^3
\left(1-\frac{\Gamma}{3H}\right)^2$, however, such a condition is by no means
necessary for the attainment of equilibrium.
