I have been a Support Scientist at the Joint Institute for VLBI
in Europe since August 2009, and obtained my PhD in the UCC Radio Astronomy Group
in early 2010. My PhD research focused on searches for transverse Faraday-rotation
gradients across AGN jets, interpreted as direct observational evidence that these
jets have helical magnetic fields.
Contours of total intensity with the Faraday rotation distribution superimposed in colour. The Faraday rotation shows a gradient across the jet, characteristic of a helical jet magnetic field. There are oppositely directed gradients in the core region and jet, which may be due to a "nested helix" magnetic-field structure, as is shown below.
I identified a number of new examples of jets with transverse Faraday-rotation gradients, as well as an entirely new phenomenon: transverse Faraday-rotation gradients that reverse either with distance from the core or with time. These Faraday-rotation gradient reversals may provide evidence for models in which the jet magnetic field emerges in the inner disc and closes in the outer disc - when the corresponding loops of magnetic field are "wound up", this should give rise to a nested helical-field structure.
The left panel shows a magnetic field that emerges in the inner accretion disc and closes in the outer accretion disc. When this type of magnetic-field structure is wound up by the differential rotation of the accretion disc, it gives rise to a "nested helical field" structure such as that show to the right.
Shane P. O' Sullivan
My PhD research was on high-resolution polarization studies of AGN jets. I studied six jets from 5 to 43 GHz with the American VLBA radio interferometer allowing Faraday rotation measure analysis on a range of scales in the jets.
Frequency-dependent shift of the VLBI core of 2007+777 relative to the core at 43 GHz. The fitted curve corresponds to the indicated value for the parameter kr for a Blandford-Konigl jet. This value is equal within the uncertainties to kr = 1, which corresponds to equipartition between the energies in the relativistic particles and the magnetic field of the jet.
The results supported the presence of ordered helical magnetic fields surrounding these jets while we found magnetic field strengths ranging from tens to hundreds of milliGauss in the compact inner jet regions.
Magnetic (B) field as a function of distance from the jet base derived from the frequency-dependent core shifts for 2007+777. The derived B field falls off roughly as r-1 with distance from the jet base. The right panel shows an extrapolation of the derived B-field dependence to much smaller scales. The extrapolated field values based on my parsec-scale VLBI measurements are consistent with predictions for magnetically-launched jets (points at 10-4 and 10-5 pc).
I'm currently working as a postdoctoral researcher at the Australian Telescope National Facility (ATNF) in Sydney on a $100 million next-generation radio telescope project known as the Australian Square Kilometre Array Pathfinder (ASKAP) being built by CSIRO in Western Australia.
The telescope will survey the sky 100 times faster than current instruments and one of the main science goals of this experiment is to address fundamental questions about the origin and evolution of magnetic fields in the Universe.