Research Areas:

Radio emision from AGNs

My primary interest is the study of the structure and evolution of the highly collimated, relativistic jet flows emanating from the central engines of active galactic nuclei. Centimeter-band flux and polarization variability is a well-documented, all-pervasive feature of extragalactic objects which provides a powerful probe for studying the physical conditions in the radioband emitting regions of these sources, and my work has used the temporally well-sampled variability data obtained with the University of Michigan 26-meter telescope to study currently-active sources. A combination of results from the UMRAO program and VLBI images with high spatial resolution, such as that shown below, have provided strong evidence for the frequent development of shocks in the parsec-scale regions of such sources and supported the existence of class-dependent differences in their jet flows. This work continues to be carried out with my collaborators Hugh Aller and Philip Hughes .

While our past work has successfully treated radio jets as hydrodynamical flows with passive magnetic fields, the magnetic energy in the region powering an AGN must be high based on theoretical considerations. As part of an effort to investigate the connection between these regimes, we resumed our program to observe circular polarization (CP) with the 26-meter paraboloid at 4.8 and 8.0 GHz in 2002, and subsequently began observations at our third primary frequency, 14.5 GHz. Our telescope is the only one in the world which is currently being used to monitor the spectral evolution of CP in AGNs. Combined with VLBA observations, these data are being used to study the magnetic field alignment in AGN radio jets, to investigate jet particle composition, and to determine whether the magnetic fields in jets have a fixed (longterm) polarity. The latter would provide evidence for a direct connection between the jet and the putative rotating black hole and/or accretion disk which launches it. Because CP emission is very weak and requires long integration times, we are concentrating on observing a group of 14 strong, active sources in all four Stokes parameters; the majority have detected CP emission from past UMRAO observations. The events studied generally last for several months, and we expect to be able to determine whether CP spectral variations are related to changes in opacity and/or to variations in total flux density and linear polarization. The most interesting events captured so far are in 3C 279 where a well-defined sign reversal has occurred at 4.8 GHz, and where we have now seen several epochs where the sign of Stokes V is not the same as all three frequencies; this behavior is direct evidence against the use of the polarity of V as an indicator of the sense of rotation of a central black hole or accretion disk in this object, and we have been able to model the spectral behavior, including the sign reversal, with a simple stochastic model. Our polarimetry studies are funded in part by the NSF under grant AST-0607523.

Two related projects in progress combine 26-meter data and high resolution, VLBA observations. The first of these is a continuing study of oblique shocks with collaborators J. Wardle (Brandeis) and D. Homan (Denison University) to determine whether the class-dependent differences found in the magnetic field orientations of BL Lacs and QSOs can be explained by a single mechanism: shocks at angles to the flow direction. A second very large, long term project combines UMRAO monitoring results with kinematic information on component motions and structural evolution, and linear and circular polarization, for a flux-limited sample of about 200 flat-spectrum sources (MOJAVE) in a collaboration lead by K. Kellermann (NRAO) and M. Lister (Purdue). Maps, movies, and a more detailed description of this survey can be found at MOJAVE .

VSOP 5 GHz map of the EGRET-detected BLazar 1633+382 obtained by J. Ulvestad, T. Vestrand, G. Stacy, and J. Biretta (figure size 8 mas by 8 mas). The core-curved jet morphology seen here is typical of that in AGNs. The study of this compact extragalactic source is one of many such objects included in the UMRAO variability program.

Studies of gamma-ray-bright and X-ray bright AGNs

Over a dozen sources are now known to emit at TeV energies from observations with Whipple, MAGIC, and other very high energy detectors. The radioband properties of four HBL BL Lac objects -- Mrk 421, Mrk 501,1959+650, and 2344+514 -- are being investigated as part of a collaboration between UMRAO and VERITAS.

Broadband variability data provide important constraints on jet properties, but relatively little variability information was available in the X-ray spectral band for blazars until the launch of RXTE. I am currently involved in the study of four blazars (BL Lac, 3C 273, 3C 279, and 1510-089) and of the radio galaxy 3C 111, using combined radio and RXTE monitoring data (with collaborators A. Marscher and S. Jorstad; Boston U.). The specific goals of this project are to investigate the X-ray emission mechanism and to identify the site of the X-ray emission. A study of the inner jet of BL Lacertae, based on optical and radio band polarimetry, broadband flux density measurements from the radio to the TeV gamma ray band, and VLBA imaging data during a recent outburst, provided evidence for the presence of an acceleration-collimation zone, a region in the jet which had only been predicted theoretically (see Nature, 452, 966, 2008). Past study of the radio galaxy 3C 120 revealed a temporal association between dips in the 2-20 keV X-ray flux and the ejection of new superluminal components identified from a series of 43 GHz VLBA maps. (See Nature, 417, pp. 625-627, 2002). Subsequent observations identified a range of behavior but continued to support the interpretation that an instability occurs in the accretion disk/corona (the origin of the X-ray emission) which injects energy into the jet, forming a shock, and emerging in the portion of the jet imaged with the radio observations. Information and an animation illustrating this discovery can be found at 3c120 . A similar association is now being sought in 3C 111, a source which exhibits persistent Fe K alpha lines, indicating that the bulk of the X-ray emission is associated with the central engine. A third radio galaxy, NGC 1052, also believed to show a accretion disk/jet connection is under study in a separate effort led by M, Kadler and K. Weaver (NASA Goddard). These X-ray projects have been funded in part by a series of NASA grants for ground-based support of satellite-borne experiments, and such studies are very important for establishing a connection between the accretion disk and the jet in AGNs.

Recent papers describing my work can be found in Selected Publications and Preprints.

Examples from the UMRAO program:

Left: Monthly-averaged Stokes parameters for 3C 345. Note the change in polarity in circular polarization during the early 1980s (top panel). Right: Weekly averages of the UMRAO data since 2005.0 for the very bright southern QSO OV-236. The top panel shows fractional circular polarization (Stokes V), the middle two panels show the linear polarization, and the bottom panel shows the total flux density. The amplitude of Stokes V in this source has ranged from 1 percent (unusually high for an AGN) to 0 percent. Contrary to the behavior we are finding in most of our program sources, the same sign of Stokes V (negative) has persisted for more than two decades and throughout many individual outbursts. Gaps in the data trains, most obvious at 4.8 GHz, correspond to times when the source is too near to the sun for observation.

Description of the UM Radio Astronomy Observatory.

Light curves showing 2-week averages for selected sources are now available at If you need data before this task is completed, please contact me at mfa@umich.edu.

Personal:

Things I like to do...

Photo courtesy of D. Harvey and Mike Wiegele helicopter skiing.

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