  GRB 130427A occurred in a relatively nearby galaxy; its prompt emission had
the largest GRB fluence ever recorded. The afterglow of GRB 130427A was bright
enough for the Nuclear Spectroscopic Telescope ARray (NuSTAR) to observe it in
the 3-79 keV energy range long after its prompt emission (~1.5 and 5 days).
This range, where afterglow observations were previously not possible, bridges
an important spectral gap. Combined with Swift, Fermi and ground-based optical
data, NuSTAR observations unambiguously establish a single afterglow spectral
component from optical to multi-GeV energies a day after the event, which is
almost certainly synchrotron radiation. Such an origin of the late-time
Fermi/LAT > 10 GeV photons requires revisions in our understanding of
collisionless relativistic shock physics.
