Department of Astronomy
University of Michigan
Ann Arbor, MI, 48109-1042
Email:email@example.com Phone: 734 / 764-3465
Dept 734 / 764-3440
FAX: 734 /763-6317
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 since 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 was funded in part by the NSF under grant AST-0607523.
A very large, long term project combines UMRAO monitoring with kinematic information at 15 GHz 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 led by M. Lister (Purdue) and K. Kellermann (NRAO). Maps, movies, and a more detailed description of this survey can be found at MOJAVE . Links to publications are also available.
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.
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 search for characteristic changes in the centimeter-band linear polarization data produced by the onset of shocks during centimeter-band outbursts temporally associated with gamma-ray flaring, and to use the radio-band data to constrain radiative transfer models incorporating propagating shocks. The UMRAO monitoring data have captured several events temporally associated with gamma-ray flaring which do exhibit the expected shock signature. We have fit these time segments of the multifrequency flux and linear polarization data with radiative transfer models allowing for propagating shocks at any angle to the jet flow direction in three sources: 0420-014, OJ 287, and 1156+295, and modeling is in progress for events in two additional sources (0727-115 and 0716+714). The required code for this work was developed by Philip Hughes in an earlier phase of the project. A sample simulation can be seen at MODEL . Preliminary modeling results were presented at the conference ``The Innermost Regions of Relativistic Jets and Their Magnetic Fields'' held in Granada, Spain in June 2013, and a Proceedings paper co-authored with P. Hughes, H. Aller, and T. Hovatta can be found at Granada. A journal paper describing the full modeling procedures and results was submitted to the Astrophysical Journal in February 2014. This work has quantitatively demonstrated that the characteristics of the spectral evolution of the linear polarization and total flux density can be reproduced by simulations incorporating propagating shocks. By comparing the simulations and the UMRAO data, we constrained the jet flow conditions during the three events modeled, determined the jet orientation, and identified the number of shocks and their parameters (strength, sense, speed, and orientation). Subsequently we modeled a second event in 1156+295 which is an orphan (radio but no gamma-ray) flare in spite of being nearly identical in centimeter-band spectral behavior to the originally-modeled outburst with temporally-associated gamma-ray flaring. Collaborative work incorporating results from the Boston University 43 GHz VLBA program attributes the difference in cross-band behavior to shock structure and interacting shocks: the radio-band events with gamma-ray counterparts show complex structure. Preliminary results will be presented at the Boston AAS meeting in June 2014 in a poster paper (number 221.09). This work continues to be made possible by a series of Fermi GI grants during cycles 2 through 6; most recently these are NNX11AO13G and NNX13AP18G.
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 galaxies 3C 120 and 3C 111, using combined radio and RXTE monitoring data (with collaborators A. Marscher and S. Jorstad; Boston U.). This project investigated the X-ray emission mechanism and site. A study of the inner jet of BL Lacertae in which I participated, based on optical and radio band polarimetry, broadband flux density measurements from the radio to the TeV gamma ray band, and VLBA imaging data 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 published in ApJ: see 1510. A picture in which an emission feature follows a spiral path through a predominantly toroidal magnetic field pattern in the acceleration and collimation zone was proposed; over the long term the X-ray and 14.5 GHz variability correlate in this source. 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 was found 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 has been studied 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.
Papers describing my work can be found in Selected Publications and Preprints.
My cv in pdf format can be found at CV
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 firstname.lastname@example.org.