  The taxonomy of optical emission detected during the critical first few
minutes after the onset of a gamma-ray burst (GRB) defines two broad classes:
prompt optical emission correlated with prompt gamma-ray emission, and early
optical afterglow emission uncorrelated with the gamma-ray emission. The
standard theoretical interpretation attributes prompt emission to internal
shocks in the ultra-relativistic outflow generated by the internal engine;
early afterglow emission is attributed to shocks generated by interaction with
the surrounding medium. Here we report on observations of a bright GRB that,
for the first time, clearly show the temporal relationship and relative
strength of the two optical components. The observations indicate that early
afterglow emission can be understood as reverberation of the energy input
measured by prompt emission. Measurements of the early afterglow reverberations
therefore probe the structure of the environment around the burst, whereas the
subsequent response to late-time impulsive energy releases reveals how earlier
flaring episodes have altered the jet and environment parameters. Many GRBs are
generated by the death of massive stars that were born and died before the
Universe was ten per cent of its current age, so GRB afterglow reverberations
provide clues about the environments around some of the first stars.
