Juan Carlos Algaba

My PhD project focuses on searches for correlations between the optical polarization and VLBI core polarization properties of core-dominant AGN. My results indicate a close connection between the optical and radio emission of some, but not all, AGN, as well as evidence for very high Faraday rotations in the cores of some quasars.

I graduated in the University of Valencia, in Spain. After a summer project in JIVE, I was informed by its staff (H. E. Bignall) of a position for a PhD in Ireland with D.C.Gabuzda and I was happy to join her group. Here, my research interests are high frequency VLBA polarization observations of AGN.
Whereas one of the group's main focuses is the morphology of the magnetic fields (looking for evidences about its helical nature), I am more interested in their origin: On what scales do the fields giving rise to the polarization we observe originate? Is this radio emission cospatial with synchrotron emission at optical wavelengths? Is there a correlation in all AGN between the optical and high frequency VLBI radio core EVPA if simultaneous measurements are compared? In the course of trying to answer those questions, I have also embarked on some side quests, such as searches for high Faraday rotation in the VLBI cores, and investigation of core shifts and spectral thin-thick transitions.

The data with which I am addressing these questions in my thesis research are 43+22+15GHz and 43+24+22+15+12GHz VLBA polarization data, obtained nearly simultaneously with optical polarization data obtained by Paul Smith at the Steward Observatory. To date, there is evidence that correlation between optical and Faraday corrected VLBA radio core polarization angle exists for BL Lacs, but only some hints of the same behaviour happening in quasars are shown. We are currently investigating if, once high or internal Faraday rotation or other physical phenomenology occouring in the inner regions of the AGN are taken into account, quasars seem to exhibit more clear evidence for this correlation; or, if they don't, being able to explain why.


The difference between the Faraday-corrected VLBA-core and optical position angle Deltachi has been found for a sample of sources (White: BL Lacs, light Grey: LPQ and dark grey: HPQ. BL Lacs seem to have a peak at a small \Delta\chi, where as the distribution of quasars seems flatter. Why is that happening?


Position angles are shown for different observed frequencies: from left to right, optical and radio 43, 22 and 15 GHz. It seems clear that the radio-data can be fitted to a straight line and Faraday rotation can be obtained. However, the optical point is clearly off. This is not due to a thin-thick transition (which would involve a rotation of 90 degrees) Is it then due to high Faraday rotation (involving ambiguities of 180 degrees), or other physical processes? We are on our way to discover it!