  A method of determining the temperature of the nonradiative reservoir in a
microcavity exciton-polariton system is developed. A general relation for the
homogeneous polariton linewidth is theoretically derived and experimentally
used in the method. In experiments with a GaAs microcavity under nonresonant
pulsed excitation, the reservoir temperature dynamics is extracted from the
polariton linewidth. Within the first nanosecond the reservoir temperature
greatly exceeds the lattice temperature and determines the dynamics of the
major processes in the system. It is shown that, for nonresonant pulsed
excitation of GaAs microcavities, the polariton Bose-Einstein condensation is
typically governed by polariton-phonon scattering, while interparticle
scattering leads to condensate depopulation.
