  The photon pair production by pion annihilation in a hot and dense medium at
the chiral phase transition is investigated within a chiral quark model. As a
direct consequence of this transition the $\sigma$ meson appears as a bound
state in the domain of temperatures and chemical potentials where the condition
$M_\sigma(T,\mu) \approx 2 M_\pi(T,\mu)$ is fulfilled. This effect results in a
strong enhancement of the cross section for the pion annihilation process $2
\pi \to 2 \gamma$ compared with the vacuum case. The calculation of the photon
pair production rate as function of the invariant mass shows a strong
enhancement and narrowing of the $\sigma$ meson resonance at threshold due to
chiral symmetry restoration.
