  Near a black hole, differential rotation of a magnetized accretion disk is
thought to produce an instability that amplifies weak magnetic fields, driving
accretion and outflow. These magnetic fields would naturally give rise to the
observed synchrotron emission in galaxy cores and to the formation of
relativistic jets, but no observations to date have been able to resolve the
expected horizon-scale magnetic-field structure. We report interferometric
observations at 1.3-millimeter wavelength that spatially resolve the linearly
polarized emission from the Galactic Center supermassive black hole,
Sagittarius A*. We have found evidence for partially ordered fields near the
event horizon, on scales of ~6 Schwarzschild radii, and we have detected and
localized the intra-hour variability associated with these fields.
