Shane O'Sullivane (USYD)

The First Broadband Circular Polarization Spectrum of an AGN Jet -- Shane O'Sullivane Colloquium

Abstract

Both the three-dimensional magnetic field structure and particle composition of AGN jets are still not well constrained. Detecting continuum circularly-polarized radio emission from AGN jets is potentially a very powerful diagnostic of the jet magnetic field and particles since, unlike linear polarization, it is expected to remain almost completely unmodified by external screens. There are many predictions for the origin of the circular polarization emission in radio-loud AGN and determining the correct model can potentially constrain the polarity of the black hole/accretion disk magnetic field as well as the black hole spin/direction of rotation of the accretion disk.

We present full-polarization observations of PKS B2126-158 from 1 to 10 GHz with the ATCA, determining for the first time, the circular polarization (CP) spectrum of a relativistic AGN jet. From 1.5 to 6.5 GHz, the percentage CP increases with frequency, implying that the emission is coming from a region with a range of optical depths, as expected for an inhomogeneous, conically self-similar jet. From 8 to 10 GHz, the percentage CP decreases rapidly with frequency, ruling out purely intrinsic synchrotron CP emission and conclusively supporting a model in which some linear polarization is converted to CP within the jet. Further study of the full-Stokes emission of PKS B2126-158, in conjunction with detailed radiative transfer simulations, will enable us to develop a robust model against which other AGN jet systems can be tested.