ATNF Marsfield Lecture Theatre

Shane O'Sullivan

Magnetic field properties of AGN jets from multi-frequency VLBA polarimetry - Shane O'Sullivan Colloquium

I will present results on multi-frequency (4.6, 5.1, 7.9, 8.9, 12.9, 15.4, 22 and 43 GHz) VLBA polarization observations of the parsec-scale region of several "blazar" jets. Observing across a wide range of frequencies, separated by short and long intervals, enabled reliable determination of the distribution of Faraday rotation on a range of jet scales and hence, the intrinsic jet magnetic field structure. Detection of transverse rotation measure (RM) gradients provided strong evidence for the presence of helical magnetic fields in the magneto-ionic material surrounding the jet. The results also show that the magnitude of the core RM increases systematically with frequency, simply implying that the electron density and/or magnetic field strength is increasing as we get closer to the central engine.

Accurate measurement of the frequency-dependent shift of the self-absorbed radio core ("core-shift") is required for multi-frequency analysis of VLBI data. The measured core-shifts were used to calculate equipartition magnetic field strengths of the order of 10's to 100's of mG in the radio jet cores from 4.6 to 43 GHz. Extrapolating the results back to the accretion disk and black hole jet-launching distances, the magnetic field strengths are found to be consistent with those expected from theoretical models of magnetically powered jets.