Helical B Fields in AGN Jets

Research done in the Extragalactic Radio Astronomy group at UCC suggests that many of the observed jet B-field structures may correspond to intrinsic, possibly helical, B fields associated with the jets themselves. Helical jet B fields could come about in a natural way due to the combination of the rotation of the central black hole and accretion disk and the jet outflow.

 

One key observational signature for helical jet B fields is connected to the phenomenon of Faraday rotation: a rotation of the plane of polarization that occurs when a polarized electromagnetic wave passes through a region with free electrons and magnetic field. The rotation comes about due to the different propagation velocities of the right-circularly polarized and left-circularly polarized components of the wave in the plasma.

The amount of rotation is determined by the density of free electrons and the line of sight component of the B field in the plasma, and is proportional to the observing wavelength squared. The sign of the Faraday rotation coincides with the sign of the line-of-sight B field.

As was pointed out by Roger Blandford in the early 1990's, if some jets carry toroidal or helical B fields, this should lead to the presence of Faraday rotation-measure gradients transverse to the jets, due to the systematic variation of the line-of-sight component of the B field across the jet. Our group has found many examples of this phenomenon, suggesting that helical B fields may be common in AGN jets.