  Gamma-ray burst (GRB) 130427A is one of the most energetic GRBs ever
observed. The initial pulse up to 2.5 s is possibly the brightest well-isolated
pulse observed to date. A fine time resolution spectral analysis shows
power-law decays of the peak energy from the onset of the pulse, consistent
with models of internal synchrotron shock pulses. However, a strongly
correlated power-law behavior is observed between the luminosity and the
spectral peak energy that is inconsistent with curvature effects arising in the
relativistic outflow. It is difficult for any of the existing models to account
for all of the observed spectral and temporal behaviors simultaneously.
