  Direct photon production from a longitudinally expanding quark-gluon plasma
(QGP) at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC)
energies is studied with a real-time kinetic description that is consistently
incorporated with hydrodynamics. Within Bjorken's hydrodynamical model, energy
nonconserving (anti)quark bremsstrahlung q(\bar{q})\to q(\bar{q})\gamma and
quark-antiquark annihilation q\bar{q}\to \gamma are shown to be the dominant
nonequilibrium effects during the transient lifetime of the QGP. For central
collisions we find a significant excess of direct photons in the range of
transverse momentum 1-2 \lesssim p_T \lesssim 5 GeV/c as compared to
equilibrium results. The photon rapidity distribution exhibits a central
plateau. The transverse momentum distribution at midrapidity falls off with a
{\em power law} p^{-\nu}_T with 2.5 \lesssim \nu \lesssim 3 as a consequence of
these energy nonconserving processes, providing a distinct experimental {\em
nonequilibrium signature}. The power law exponent \nu increases with the
initial temperature of the QGP and hence with the total multiplicity rapidity
distribution dN_\pi/dy.
