MARGO F. ALLER Department of Astronomy University of Michigan Ann Arbor, MI, 48109-1042 Email:mfa@umich.edu Phone: 734 / 764-3465 Dept 734 / 764-3440 FAX: 734 /763-6317

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. This program continued through June 30, 2012 providing a decade of these unique measurements: our telescope was the only one in the world dedicated to monitoring 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 concentrated on observing a group of 14 strong, active sources in all four Stokes parameters. The events studied generally last for several months, and we are investigating 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 are in 3C 279 where sign reversals occurred at 4.8 GHz; this behavior appeared to argue against the use of the polarity of V as an indicator of the sense of rotation of a central black hole or accretion disk, but it may be the result of opacity effects since we have only seen sign reversals in any of our program sources at our two lower frequencies. We have been able to model the spectral behavior, including the sign reversal, with a simple stochastic model. This work is funded in part by the NSF under grant AST-0607523.

A very large, long term project combines UMRAO monitoring with kinematic information on component motions and structural evolution, and linear and circular polarization, for a flux-limited sample of about 300 flat-spectrum sources (MOJAVE) in a collaboration lead by M. Lister (Purdue) and K. Kellermann (NRAO). Maps, movies, and a more detailed description of this survey can be found at MOJAVE .

VSOP 5 GHz map of the GeV 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 AGN. 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 AGN

From September 2009 through June 30, 2012 we intensively monitored the linear polarization and total flux density of a sample of about 2 dozen gamma-ray-bright sources detected by Fermi in order to identify properties in the radio jet associated with gamma-ray flaring. Our specific goals are to identify changes in the magnetic field direction associated with gamma-ray flares, and to test whether gamma-ray flaring is associated with the onset of shocks, a plausible particle acceleration mechanism. The data in hand reveal several events temporally associated with gamma-ray flaring which show the expected shock signature. Our next step is to fit the multifrequency flux and linear polarization data with newly-developed radiative transfer models allowing for propagating shocks at any angle to the jet flow direction. The required code, incorporating retarded time effects, has been developed by Philip Hughes, and is in the testing stage. A sample simulation can be seen at MODEL .

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 was 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 goals of this project were to investigate the X-ray emission mechanism and site. 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). An analysis of broadband data for PKS 1510-089 obtained during 2008.6 to 2009.5, including Fermi gamma-ray fluxes, was recently published in ApJ: see 1510. A picture in which an emission feature follows a spiral path through a predominantly torroidal magnetic field pattern in the acceleration and collimation zone is proposed; over the long term the X-ray and 14.5 GHz variability correlate in this source. Earlier work on 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). 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 (University of Erlangen-Nuremberg, Germany) and K. Weaver (NASA Goddard). These high energy projects were funded in part by a series of NASA grants. A new study of 3C 120 (led by Chris Reynolds, Univ. of Maryland) using 2012 SWIFT monitoring data is in progress.

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

My cv in pdf format can be found at CV

Examples from the UMRAO Circular Polarization Program:

Left: Monthly-averaged total flux density and circular polarization 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.

Light curves showing 2-week averages for selected sources are now available on the Department of Astronomy web site under Facilities (Data Sets) If you need data for additional sources, please contact me at mfa@umich.edu.