A fraction of all active galactic nuclei (AGN) shows extended, highly collimated outflows of gas – jets – from their centers. This phenomenon is probably caused by an interplay of black hole rotation, accreted matter, and magnetic fields (the Blandford–Znajek mechanism). For a rapidly spinning Kerr black hole, the innermost stable circular orbit, and thus the inner edge of the accretion disk, can be located within the ergosphere. This permits the transfer of rotational energy from the black hole to the accreted matter. In addition, accretion disks are likely to be permeated by magnetic fields. Within the ergosphere, magnetic field lines will be twisted by frame-dragging, thus forming “tunnels” or “chimneys” made of magnetic field lines. A part of the material in the innermost parts of the accretion disk will be ejected along those magnetic “tunnels”; the magnetic field keeps the material collimated even over large distances, leading to jets ranging over a few megaparsecs in the most extreme cases. However, despite their importance, none of the key features of AGN jets – their launching, collimation, propagation, and interaction with interstellar gas – is understood. AGN jets are objects of ongoing research and their physical properties are hotly debated.
The PAGaN project analyzes the physical – especially plasma-physical – properties of selected AGN via multi-frequency polarization monitoring with KVN. It observes polarization both as function of time and frequency, thus constraining the evolution and propagation of shocks, particle densities, and magnetic field geometries.
PAGaN exploits the unique capability of KVN to observe the full polarization state of a source at four cm-to-mm frequencies spanning a factor six in frequency. PAGaN observes seven radio-bright AGN twice per year and uses dual-frequency phase referencing to improve the quality of the 86 and 129-GHz data. For each source, linear polarization maps are constructed and rotation measures as function of frequency are calculated.
Throughout 2016, a limited number of observations has been obtained in order to test the feasibility of the project. After a successful proof-of-concept, regular observations will begin in 2017. The KVN data will be complemented by archival VLBA, SMA, optical, and gamma-ray data to provide a multiwavelength view on the selected targets.
BL Lac, 3C 111, 3C 120, 3C 273, 3C 345, 4C +11.69, 4C +21.35
The PAGaN collaboration include the following people (in alphabetical order):