Some of the fundamental properties of the jets in Active Galactic Nuclei (AGN), and of their physical connection to the massive black holes and accretion disks which power and launch them, remain enigmatic. This project involves the study of the small component of circularly polarized radio frequency emission, which can provide important clues concerning the topology and role of the magnetic fields in AGN evolution, and the nature of the relativistic particles, in the emitting regions. This work uses extensive multi-frequency polarimetry at the University of Michigan 26-meter paraboloid, combined with selected observations at other facilities, to study the time variability of polarized radiation and compare it with radiative transfer models. These comparisons will determine the amount of order in the magnetic fields and the true identity of the emitting particles, which is central to understanding the environments of massive black holes and the mechanisms by which energy is extracted from them.
The large and expanding database of unique measurements of active objects during outburst will continue to be made readily available to other researchers for independent scientific studies. Undergraduate and graduate students, including some from other institutions, gain important experience by working directly with data, and the Michigan radio telescope continues to provide critical training for future astronomers.
Radio-band circular polarization observations can provide unique information on the properties of the magnetic field embedded in the relativistic jets emanating from the central black holes of Active Galactic Nuclei (AGN), and on the particle content of these jets when compared with model predictions. Such jets are common, but basic questions about them, including their particle content and the role of the magnetic field in their launching and development, remain unanswered. While circular polarization observations can provide important constraints, they are not generally undertaken because the weak levels of emission make it difficult to obtain significant detections. In this project, circular polarization monitoring observations at three centimeter-band frequencies (4.8, 8.0 and 14.5 GHz) were obtained over nearly a decade for a core group of 12 radio-bright blazars (AGN with highly-variable flux across the electromagnetic spectrum); these objects have relativistic jets pointed nearly toward the observer and flux levels enhanced by beaming. Several additional blazars were also observed in this project during flux outbursts (flares) only. The data obtained from the program consist of simultaneous, source-integrated measurements of the total flux density, linear polarization, and circular polarization using the University of Michigan 26-meter telescope equipped with polarimeters built and designed in-house. The project goals were to determine the spectral evolution of the circular polarization, the range of changes of the amplitude of the emission, the time scale for the variations, the polarity of the circular polarization, and the relation of the variations in the circular polarization to those in total flux density and linear polarization. The circular polarization was found to vary in amplitude with time in several of the sources and to reach a maximum level of one percent of the total flux density. Typical circular polarization amplitudes in AGN are only a few tenths of a percent or less. The circularly polarized emission was found to be stochastic in nature as there were observing epochs during flaring when no circular polarization was detected. The timescales of the variations in the circularly polarized emission were generally several weeks to months, and the emission process identified from the behavior patterns in the data was linear-to-circular mode conversion. No direct relation was found between the amplitudes in linear polarization and circular polarization, but the circularly polarized emission was generally observed when the total flux density spectrum showed evidence for self-absorption within the emitting region. While a preferred polarity (positive or negative) was found for most program sources, polarity changes were observed in several sources at the two lower observing frequencies (4.8 and 8.0 GHz). These changes persisted over several weeks to a few months, and at some epochs, polarity differences were found as a function of frequency. This contrasts with the behavior at 14.5 GHz where no evidence for a polarity change was found above the 3-sigma detection level. The presence of a long-term preferred polarity is of interest since it may be an indicator of the direction of rotation of the central black hole and associated accretion disk, according to some theories. The observed polarity changes with time potentially argue against the use of the sign of the circular polarization as an indicator of the sense of rotation of the central black hole and associated accretion disk. However, these changes may instead be indicators of the conditions in a region of the jet where the underlying magnetic field direction is masked by turbulence, or they may be a measure of the conditions associated with flaring rather than of physical conditions in the quiescent jet. The primary product from the project is a unique data set. To our knowledge these data are the only long-term observations of spectral variability including circular polarization for AGN. These measurements, in combination with models, are currently being used to further define AGN jet properties.
