Enno Middelberg (Max-Planck-Institut für Radioastronomie)

Gas and magnetic fields in AGN surroundings, and a new phase-referencing technique for 86 GHz VLBI - Enno Middelberg Colloquium

ATNF Marsfield Lecture Theatre

Abstract

We have a project to measure magnetic fields in the obscuring tori of
AGNs. We observed five free-free absorbed jets in radio galaxies with
polarimetric VLBI at 15 GHz to determine Faraday rotation
measures. Surprisingly, all sources are unpolarized, putting severe
constraints on the degree of magnetic field turbulence and the gas
distribution in these objects. We have further carried out
multi-frequency VLBI monitoring of the Seyfert 2 galaxy NGC 3079 and
find unusually steep and inverted spectra in the radio components. We
derive general properties of Seyfert galaxies using published VLBI
observations of Seyfert galaxies.

We have developed a new phase-referencing technique for high frequency
VLBI observations with the VLBA. Instead of inserting short scans on a
calibrator into the target source observations, the target source is
continuously observed while rapidly switching between the target
frequency and a lower reference frequency. We demonstrate that the
technique allows phase calibration almost reaching the thermal noise
limit and present the first detection of the AGN in NGC 4261 at 86
GHz. This is the weakest source ever detected with VLBI at that
frequency.