  The structure of the one loop self-energy graphs of the $\rho$ meson is
analyzed in the real time formulation of thermal field theory. The modified
spectral function of the $\rho$ meson in hot hadronic matter leads to a large
enhancement of lepton pair production below the bare peak of the $\rho$. It has
been shown that the effective temperature extracted from the inverse slope of
the transverse momentum distributions for various invariant mass ($M$) windows
of the pair can be used as an efficient tool to characterize different phases
of the evolving matter.
