Discovery of the bow shock of Cygnus A

Rotation measure images of Cygnus A indicate that a bow shock precedes the supersonic advance of hot spot B into the intergalactic medium. The shock is radio quiet and is observed only by the rotation measure discontinuity which occurs at the point where the fields and particles in the IGM are compressed by the shock. The fact that this discontinuity is projected onto part of the source provides information on the three-dimensional structure of the radio source and supports models of extragalactic radio sources in which the jet varies direction on relatively short time scales. From the observed rotation measures, we calculate magnetic field strengths in the cluster gas of ~ 7.5 μG.