  Observations of the afterglows of gamma-ray bursts (GRBs) have revealed that
they lie at cosmological distances, and so correspond to the release of an
enormous amount of energy. The nature of the central engine that powers these
events and the prompt gamma-ray emission mechanism itself remain enigmatic
because, once a relativistic fireball is created, the physics of the afterglow
is insensitive to the nature of the progenitor. Here we report the discovery of
linear polarization in the prompt gamma-ray emission from GRB021206, which
indicates that it is synchrotron emission from relativistic electrons in a
strong magnetic field. The polarization is at the theoretical maximum, which
requires a uniform, large-scale magnetic field over the gamma-ray emission
region. A large-scale magnetic field constrains possible progenitors to those
either having or producing organized fields. We suggest that the large magnetic
energy densities in the progenitor environment (comparable to the kinetic
energy densities of the fireball), combined with the large-scale structure of
the field, indicate that magnetic fields drive the GRB explosion.
