  Ultrarelativistic collisions of large nuclei produce a short-lived plasma of
deconfined quarks and gluons. Of all the GeV-energy photons detected in these
nuclear collisions, only a small number are emitted directly by the quark-gluon
plasma. We use the characteristic near-exponential energy spectrum of these
photons to estimate the maximum temperature of the plasma, assuming it
undergoes hydrodynamic expansion. We quantify non-exponential corrections to
the photon spectrum resulting from the characteristic equation of state and
photon emission rate of nuclear matter.
