  We use holographic methods to show that photons emitted by a strongly coupled
plasma subject to a magnetic field are linearly polarized regardless of their
four-momentum, except when they propagate along the field direction. The
gravitational dual is constructed using a 5D truncation of 10-dimensional type
IIB supergravity, and includes a scalar field in addition to the constant
magnetic one. In terms of the geometry of the collision experiment that we
model, our statement is that any photon produced there has to be in its only
polarization state parallel to the reaction plane.
