Abstract
In theoretical models for the electromagnetic launching of astrophysical jets, a helical magnetic (B)-field component is generated through the winding up of an initial longitudinal field component by the rotation of the cental black hole and accretion disk. This helical field component travels outward with the jet plasma. There is now abundant evidence that the jets of active galactic nuclei carry helical B fields, and the presence of such fields has been invoked to explain a wide range of phenomena observed in these jets. However, distinguishing between features associated with this inherent jet B field and with B fields generated by local phenomena such as shocks and shear can be challenging. There is now evidence that the field that is accreted is dipolar like, giving rise to a current distribution with inward currents along both jet axes and outward currents in a more extended region around the jets. Striking limb brightening has been observed for several relatively nearby active galactic nuclei; it is argued that this must be due to some intrinsic property of the jet, which is independent of the viewing angle, such as its helical B field, or mass loading and/or particle acceleration at the jet edges. Circular-polarization observations may make it possible to reconstruct the full three-dimensional B field of jets carrying a helical B-field component, and to correctly infer the direction of rotation of the central black hole and its accretion disk.
Highlights
For reasons that are not clear, in a significant minority of active galactic nuclei (AGNs), the accretion process leads to the ejection of “jets” of relativistic plasma from a region near the central black hole
This strongly suggests that the origin of limb brighening is instead related to some intrinsic property of a jet propagating through an ambient medium, such as a helical B field, mass loading or particle acceleration at the jet edges
Magnetic fields of a few tenths of a Gauss, consistent with estimates of core B fields based on the frequency-dependent position of the very long baseline interferometry (VLBI) core [70,71,72,73], and degrees of order of the fields of order 10%, consistent with typical observed degrees of linear polarization, yield degrees of circular polarization (CP) mc of no more than a few tenths of a percent
Summary
While the radiation of most galaxies is dominated by thermal emission from stars and gas, the luminosities of a small fraction of “active” galaxies are vastly dominated by non-thermal radiation in compact regions in their nuclei. The fact that the radio emission is synchrotron radiation is confirmed by its appreciable degree of linear polarization, which can approach 40–50% in individual jet features (e.g., [2]) This is fairly close to the theoretical maximum degree of linear polarization for synchrotron radiation from a region with perfectly ordered B fields and a random pitch-angle distribution for the relativistic electrons [3], '75%, indicating that the jet B fields can sometimes be very well ordered. The direction of the observed plane of linear polarization, or electric vector position angle (EVPA) is orthogonal to the synchrotron B field in optically thin emission regions, and parallel to the synchrotron B field in sufficiently optically thick regions It has often been assumed in the past that this transition from optically thin to optically thick occurs near an optical depth τ ' 1. It can be challenging to discern from observations which observed features are predominantly associated with such local effects, and which reflect the helical/toroidal B fields produced when the jets are launched
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