| Hello, my name is Ronan Murphy, I am an undergrad at University College Cork in Ireland, I am just about to enter my final year. For my summer student project I worked under the supervision of Mehreen Mahmud. My task was to reduce VLBA data from December 2008, which included re-observations of 7 AGN, at 4.6 – 43 GHz, from a sample of >1 Jy BL Lac objects which showed interesting rotation measure (RM) gradient reversal patterns.
The image above shows rotation measure maps of 0716+714 at two epochs at the same frequencies of 4.6, 5.1, 7.9, 8.9, 12.0, 15.4 GHz. The left map corresponds to observations on March 2004 (Mahmud, PhD Thesis), and the right map to observations on December 2008. Each map has been convolved with its own beam. Clearly visible in both maps are rotation measure gradients increasing toward the right below the core, and also gradients increasing toward the left above the core (however, the rotation measure values have slightly changed across the epochs). This is an example of a gradient reversing (or “flipping”) over distance from the core. Clearly, this shows the rotation measure gradient reversal observed in the previous epoch is not a random, isolated phenomenon. These gradient reversals have in fact been interpreted as corresponding to a nested helical magnetic field structure arising as a natural consequence of the magnetic tower model (Lynden-Bell, 1996), where the magnetic field forms meridional loops that are anchored in the inner and outer parts of the accretion disk, which become twisted due to the differential rotation of the disc. This should give rise to an inner and an outer helical B-field which would be associated with oppositely detected RM gradients.